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diff --git a/docs/ietf/draft-ietf-bmwg-mlrsearch-02.md b/docs/ietf/draft-ietf-bmwg-mlrsearch-02.md deleted file mode 100644 index fef146618c..0000000000 --- a/docs/ietf/draft-ietf-bmwg-mlrsearch-02.md +++ /dev/null @@ -1,1359 +0,0 @@ ---- -title: Multiple Loss Ratio Search for Packet Throughput (MLRsearch) -abbrev: Multiple Loss Ratio Search -docname: draft-ietf-bmwg-mlrsearch-02 -date: 2022-03-07 - -ipr: trust200902 -area: ops -wg: Benchmarking Working Group -kw: Internet-Draft -cat: info - -coding: us-ascii -pi: # can use array (if all yes) or hash here - toc: yes - sortrefs: # defaults to yes - symrefs: yes - -author: - - - ins: M. Konstantynowicz - name: Maciek Konstantynowicz - org: Cisco Systems - role: editor - email: mkonstan@cisco.com - - - ins: V. Polak - name: Vratko Polak - org: Cisco Systems - email: vrpolak@cisco.com - -normative: - RFC2544: - -informative: - FDio-CSIT-MLRsearch: - target: https://s3-docs.fd.io/csit/rls2110/report/introduction/methodology_data_plane_throughput/methodology_data_plane_throughput.html#mlrsearch-tests - title: "FD.io CSIT Test Methodology - MLRsearch" - date: 2021-11 - PyPI-MLRsearch: - target: https://pypi.org/project/MLRsearch/0.4.0/ - title: "MLRsearch 0.4.0, Python Package Index" - date: 2021-04 - ---- abstract - -TODO: Update after all sections are ready. - -This document proposes changes to [RFC2544], specifically to packet -throughput search methodology, by defining a new search algorithm -referred to as Multiple Loss Ratio search (MLRsearch for short). Instead -of relying on binary search with pre-set starting offered load, it -proposes a novel approach discovering the starting point in the initial -phase, and then searching for packet throughput based on defined packet -loss ratio (PLR) input criteria and defined final trial duration time. -One of the key design principles behind MLRsearch is minimizing the -total test duration and searching for multiple packet throughput rates -(each with a corresponding PLR) concurrently, instead of doing it -sequentially. - -The main motivation behind MLRsearch is the new set of challenges and -requirements posed by NFV (Network Function Virtualization), -specifically software based implementations of NFV data planes. Using -[RFC2544] in the experience of the authors yields often not repetitive -and not replicable end results due to a large number of factors that are -out of scope for this draft. MLRsearch aims to address this challenge -in a simple way of getting the same result sooner, so more repetitions -can be done to describe the replicability. - ---- middle - -{::comment} - As we use kramdown to convert from markdown, - we use this way of marking comments not to be visible in rendered draft. - https://stackoverflow.com/a/42323390 - If other engine is used, convert to this way: - https://stackoverflow.com/a/20885980 -{:/comment} - -# Terminology - -TODO: Update after most other sections are updated. - -{::comment} - The following is probably not needed (or defined elsewhere). - - * Frame size: size of an Ethernet Layer-2 frame on the wire, including - any VLAN tags (dot1q, dot1ad) and Ethernet FCS, but excluding Ethernet - preamble and inter-frame gap. Measured in bytes (octets). - * Packet size: same as frame size, both terms used interchangeably. - * Device Under Test (DUT): In software networking, "device" denotes a - specific piece of software tasked with packet processing. Such device - is surrounded with other software components (such as operating system - kernel). It is not possible to run devices without also running the - other components, and hardware resources are shared between both. For - purposes of testing, the whole set of hardware and software components - is called "system under test" (SUT). As SUT is the part of the whole - test setup performance of which can be measured by [RFC2544] methods, - this document uses SUT instead of [RFC2544] DUT. Device under test - (DUT) can be re-introduced when analysing test results using whitebox - techniques, but this document sticks to blackbox testing. - * System Under Test (SUT): System under test (SUT) is a part of the - whole test setup whose performance is to be benchmarked. The complete - test setup contains other parts, whose performance is either already - established, or not affecting the benchmarking result. - * Bi-directional throughput tests: involve packets/frames flowing in - both transmit and receive directions over every tested interface of - SUT/DUT. Packet flow metrics are measured per direction, and can be - reported as aggregate for both directions and/or separately - for each measured direction. In most cases bi-directional tests - use the same (symmetric) load in both directions. - * Uni-directional throughput tests: involve packets/frames flowing in - only one direction, i.e. either transmit or receive direction, over - every tested interface of SUT/DUT. Packet flow metrics are measured - and are reported for measured direction. - * Packet Throughput Rate: maximum packet offered load DUT/SUT forwards - within the specified Packet Loss Ratio (PLR). In many cases the rate - depends on the frame size processed by DUT/SUT. Hence packet - throughput rate MUST be quoted with specific frame size as received by - DUT/SUT during the measurement. For bi-directional tests, packet - throughput rate should be reported as aggregate for both directions. - Measured in packets-per-second (pps) or frames-per-second (fps), - equivalent metrics. - * Bandwidth Throughput Rate: a secondary metric calculated from packet - throughput rate using formula: bw_rate = pkt_rate * (frame_size + - L1_overhead) * 8, where L1_overhead for Ethernet includes preamble (8 - octets) and inter-frame gap (12 octets). For bi-directional tests, - bandwidth throughput rate should be reported as aggregate for both - directions. Expressed in bits-per-second (bps). - * TODO do we need this as it is identical to RFC2544 Throughput? - Non Drop Rate (NDR): maximum packet/bandwidth throughput rate sustained - by DUT/SUT at PLR equal zero (zero packet loss) specific to tested - frame size(s). MUST be quoted with specific packet size as received by - DUT/SUT during the measurement. Packet NDR measured in - packets-per-second (or fps), bandwidth NDR expressed in - bits-per-second (bps). - * TODO if needed, reformulate to make it clear there can be multiple rates - for multiple (non-zero) loss ratios. - : Partial Drop Rate (PDR): maximum packet/bandwidth throughput rate - sustained by DUT/SUT at PLR greater than zero (non-zero packet loss) - specific to tested frame size(s). MUST be quoted with specific packet - size as received by DUT/SUT during the measurement. Packet PDR - measured in packets-per-second (or fps), bandwidth PDR expressed in - bits-per-second (bps). - * TODO: Refer to FRMOL instead. - Maximum Receive Rate (MRR): packet/bandwidth rate regardless of PLR - sustained by DUT/SUT under specified Maximum Transmit Rate (MTR) - packet load offered by traffic generator. MUST be quoted with both - specific packet size and MTR as received by DUT/SUT during the - measurement. Packet MRR measured in packets-per-second (or fps), - bandwidth MRR expressed in bits-per-second (bps). - * TODO just keep using "trial measurement"? - Trial: a single measurement step. See [RFC2544] section 23. - * TODO already defined in RFC2544: - Trial duration: amount of time over which packets are transmitted - in a single measurement step. -{:/comment} -{::comment} -{:/comment} - -* TODO: The current text uses Throughput for the zero loss ratio load. - Is the capital T needed/useful? -* DUT and SUT: see the definitions in https://gerrit.fd.io/r/c/csit/+/35545 -* Traffic Generator (TG) and Traffic Analyzer (TA): see - https://datatracker.ietf.org/doc/html/rfc6894#section-4 - TODO: Maybe there is an earlier RFC? -* Overall search time: the time it takes to find all required loads within - their precision goals, starting from zero trials measured at given - DUT configuration and traffic profile. -* TODO: traffic profile? -* Intended load: https://datatracker.ietf.org/doc/html/rfc2285#section-3.5.1 -* Offered load: https://datatracker.ietf.org/doc/html/rfc2285#section-3.5.2 -* Maximum offered load (MOL): see - https://datatracker.ietf.org/doc/html/rfc2285#section-3.5.3 -* Forwarding rate at maximum offered load (FRMOL) - https://datatracker.ietf.org/doc/html/rfc2285#section-3.6.2 -* Trial Loss Count: the number of frames transmitted - minus the number of frames received. Negative count is possible, - e.g. when SUT duplicates some frames. -* Trial Loss Ratio: ratio of frames received relative to frames - transmitted over the trial duration. - For bi-directional throughput tests, the aggregate ratio is calculated, - based on the aggregate number of frames transmitted and received. - If the trial loss count is negative, its absolute value MUST be used - to keep compliance with RFC2544. -* Safe load: any value, such that trial measurement at this (or lower) - intended load is correcrly handled by both TG and TA, regardless of SUT behavior. - Frequently, it is not known what the safe load is. -* Max load (TODO rename?): Maximal intended load to be used during search. - Benchmarking team decides which value is low enough - to guarantee values reported by TG and TA are reliable. - It has to be a safe load, but it can be lower than a safe load estimate - for added safety. - See the subsection on unreliable test equipment below. - This value MUST NOT be higher than MOL, which itself MUST NOT - be higher than Maximum Frame Rate - https://datatracker.ietf.org/doc/html/rfc2544#section-20 -* Min load: Minimal intended load to be used during search. - Benchmarking team decides which value is high enough - to guarantee the trial measurement results are valid. - E.g. considerable overall search time can be saved by declaring SUT - faulty if min load trial shows too high loss rate. - Zero frames per second is a valid min load value -* Effective loss ratio: a corrected value of trial loss ratio - chosen to avoid difficulties if SUT exhibits decreasing loss ratio - with increasing load. It is the maximum of trial loss ratios - measured at the same duration on all loads smaller than (and including) - the current one. -* Target loss ratio: a loss ratio value acting as an input for the search. - The search is finding tight enough lower and upper bounds in intended load, - so that the measurement at the lower bound has smaller or equal - trial loss ratio, and upper bound has strictly larger trial loss ratio. - For the tightest upper bound, the effective loss ratio is the same as - trial loss ratio at that upper bound load. - For the tightest lower bound, the effective loss ratio can be higher - than the trial loss ratio at that lower bound, but still not larger - than the target loss ratio. -* TODO: Search algorithm. -* TODO: Precision goal. -* TODO: Define a "benchmarking group". -* TODO: Upper and lower bound. -* TODO: Valid and invalid bound? -* TODO: Interval and interval width? - -TODO: Mention NIC/PCI bandwidth/pps limits can be lower than bandwidth of medium. - -# Intentions of this document - -{::comment} - Instead of talking about DUTs being non-deterministic - and vendors "gaming" in order to get better Throughput results, - Maciek and Vratko currently prefer to talk about result repeatability. -{:/comment} - -The intention of this document is to provide recommendations for: -* optimizing search for multiple target loss ratios at once, -* speeding up the overall search time, -* improve search results repeatability and comparability. - -No part of RFC2544 is intended to be obsoleted by this document. - -{::comment} - This document may contain examples which contradict RFC2544 requirements - and suggestions. - That is not an ecouragement for benchmarking groups - to stop being compliant with RFC2544. -{:/comment} - -# RFC2544 - -## Throughput search - -It is useful to restate the key requirements of RFC2544 -using the new terminology (see section Terminology). - -The following sections of RFC2544 are of interest for this document. - -* https://datatracker.ietf.org/doc/html/rfc2544#section-20 - Mentions the max load SHOULD not be larget than the theoretical - maximum rate for the frame size on the media. - -* https://datatracker.ietf.org/doc/html/rfc2544#section-23 - Lists the actions to be done for each trial measurement, - it also mentions loss rate as an example of trial measurement results. - This document uses loss count instead, as that is the quantity - that is easier for the current test equipment to measure, - e.g. it is not affected by the real traffic duration. - TODO: Time uncertainty again. - -* https://datatracker.ietf.org/doc/html/rfc2544#section-24 - Mentions "full length trials" leading to the Throughput found, - as opposed to shorter trial durations, allowed in an attempt - to "minimize the length of search procedure". - This document talks about "final trial duration" and aims to - "optimize overal search time". - -* https://datatracker.ietf.org/doc/html/rfc2544#section-26.1 - with https://www.rfc-editor.org/errata/eid422 - finaly states requirements for the search procedure. - It boils down to "increase intended load upon zero trial loss - and decrease intended load upon non-zero trial loss". - -No additional constraints are placed on the load selection, -and there is no mention of an exit condition, e.g. when there is enough -trial measurements to proclaim the largest load with zero trial loss -(and final trial duration) to be the Throughput found. - -{::comment} - The following section is probably not useful enough. - - ## Generalized search - - Note that the Throughput search can be restated as a "conditional - load search" with a specific condition. - - "increase intended load upon trial result satisfying the condition - and decrease intended load upon trial result not satisfying the condition" - where the Throughput condition is "trial loss count is zero". - - This works for any condition that can be evaluated from a single - trial measurement result, and is likely to be true at low loads - and false at high loads. - - MLRsearch can incorporate multiple different conditions, - as long as there is total ligical ordering between them - (e.g. if a condition for a target loss ratio is not satisfied, - it is also not satisfied for any other codition which uses - larger target loss ratio). - - TODO: How to call a "load associated with this particular condition"? -{:/comment} - -{::comment} - - TODO: Not sure if this subsection is needed an where. - - ## Simple bisection - - There is one obvious and simple search algorithm which conforms - to throughput search requirements: simple bijection. - - Input: target precision, in frames per second. - - Procedure: - - 1. Chose min load to be zero. - 1. No need to measure, loss count has to be zero. - 2. Use the zero load as the current lower bound. - 2. Chose max load to be the max value allowed by bandwidth of the medium. - 1. Perform a trial measurement (at the full length duration) at max load. - 2. If there is zero trial loss count, return max load as Throughput. - 3. Use max load as the current upper bound. - 3. Repeat until the difference between lower bound and upper bound is - smaller or equal to the precision goal. - 1. If it is not larget, return the current lower bound as Throughput. - 2. Else: Chose new load as the arithmetic average of lower and upper bound. - 3. Perform a trial measurement (at the full length duration) at this load. - 4. If the trial loss rate is zero, consider the load as new lower bound. - 5. Else consider the load as the new upper bound. - 6. Jump back to the repeat at 3. - - Another possible stop condition is the overal search time so far, - but that is not really a different condition, as the time for search to reach - the precision goal is just a function of precision goal, trial duration - and the difference between max and min load. - - While this algorithm can be accomodated to search for multiple - target loss ratios "at the same time (see somewhere below), - it is still missing multiple improvement which give MLRsearch - considerably better overal search time in practice. - -{:/comment} - -# Problems - -## Repeatability and Comparability - -RFC2544 does not suggest to repeat Throughput search, -{::comment}probably because the full set of tests already takes long{:/comment} -and from just one Throughput value, it cannot be determined -how repeatable that value is (how likely it is for a repeated Throughput search -to end up with a value less then the precision goal away from the first value). - -Depending on SUT behavior, different benchmark groups -can report significantly different Througput values, -even when using identical SUT and test equipment, -just because of minor differences in their search algorithm -(e.g. different max load value). - -While repeatability can be addressed by repeating the search several times, -the differences in the comparability scenario may be systematic, -e.g. seeming like a bias in one or both benchmark groups. - -MLRsearch algorithm does not really help with the repeatability problem. -This document RECOMMENDS to repeat a selection of "important" tests -ten times, so users can ascertain the repeatability of the results. - -TODO: How to report? Average and standard deviation? - -Following MLRsearch algorithm leaves less freedom for the benchmark groups -to encounter the comparability problem, -alghough more research is needed to determine the effect -of MLRsearch's tweakable parameters. - -{::comment} - Possibly, the old DUTs were quite sharply consistent in their performance, - and/or precision goals were quite large in order to save overal search time. - - With software DUTs and with time-efficient search algorithms, - nowadays the repeatability of Throughput can be quite low, - as in standard deviation of repeated Througput results - is considerably higher than the precision goal. -{:/comment} - -{::comment} - TODO: Unify with PLRsearch draft. - TODO: No-loss region, random region, lossy region. - TODO: Tweaks with respect to non-zero loss ratio goal. - TODO: Duration dependence? - - Both RFC2544 and MLRsearch return Throughput somewhere inside the random region, - or at most the precision goal below it. -{:/comment} - -{::comment} - TODO: Make sure this is covered elsewhere, then delete. - - ## Search repeatability - - The goal of RFC1242 and RFC2544 is to limit how vendors benchmark their DUTs, - in order to force them to report values that have higher chance - to be confirmed by independent benchmarking groups following the same RFCs. - - This works well for deterministic DUTs. - - But for non-deterministic DUTs, the RFC2544 Throughput value - is only guaranteed to fall somewhere below the lossy region (TODO define). - It is possible to arrive at a value positioned likely high in the random region - at the cost of increased overall search duration, - simply by lowering the load by very small amounts (instead of exact halving) - upon lossy trial and increasing by large amounts upon lossless trial. - - Prescribing an exact search algorithm (bisection or MLRsearch or other) - will force vendors to report less "gamey" Throughput values. -{:/comment} - -{::comment} - ## Extensions - - The following two sections are probably out of scope, - as they does not affect MLRsearch design choices. - - ### Direct and inverse measurements - - TODO expand: Direct measurement is single trial measurement, - with predescribed inputs and outputs turned directly into the quality of interest - Examples: - Latency https://datatracker.ietf.org/doc/html/rfc2544#section-26.2 - is a single direct measurement. - Frame loss rate https://datatracker.ietf.org/doc/html/rfc2544#section-26.3 - is a sequence of direct measurements. - - TODO expand: Indirect measurement aims to solve an "inverse function problem", - meaning (a part of) trial measurement output is prescribed, and the quantity - of interest is (derived from) the input parameters of trial measurement - that achieves the prescribed output. - In general this is a hard problem, but if the unknown input parameter - is just one-dimensional quantity, algorithms such as bisection - do converge regardless of outputs seen. - We call any such algorithm examining one-dimensional input as "search". - Of course, some exit condition is needed for the search to end. - In case of Throughput, bisection algorithm tracks both upper bound - and lower bound, with lower bound at the end of search is the quantity - satisfying the definition of Throughput. - - ### Metrics other than frames - - TODO expand: Small TCP transaction can succeed even if some frames are lost. - - TODO expand: It is possible for loss ratio to use different metric than load. - E.g. pps loss ratio when traffic profile uses higher level transactions per second. - - ### TODO: Stateful DUT - - ### TODO: Stateful traffic -{:/comment} - -## Non-Zero Target Loss Ratios - -https://datatracker.ietf.org/doc/html/rfc1242#section-3.17 -defines Throughput as: - The maximum rate at which none of the offered frames - are dropped by the device. - -and then it says: - Since even the loss of one frame in a - data stream can cause significant delays while - waiting for the higher level protocols to time out, - it is useful to know the actual maximum data - rate that the device can support. - -{::comment} - - While this may still be true for some protocols, - research has been performed... - - TODO: Add this link properly: https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Y.1541-201112-I!!PDF-E&type=items - TODO: List values from that document, from 10^-3 to 4*10^-6. - - ...on other protocols and use cases, - resulting in some small but non-zero loss ratios being considered - as acceptable. Unfortunately, the acceptable value depends on use case - and properties such as TCP window size and round trip time, - so no single value of target loss rate (other than zero) - is considered to be universally applicable. - -{:/comment} - -New "software DUTs" (traffic forwarding programs running on -commercial-off-the-shelf compute server hardware) frequently exhibit quite -low repeatability of Throughput results per above definition. - -This is due to, in general, throughput rates of software DUTs (programs) -being sensitive to server resource allocation by OS during runtime, -as well as any interrupts or blocking of software threads involved -in packet processing. - -To deal with this, this document recommends discovery of multiple throughput rates of interest for software DUTs that run on general purpose COTS servers (with x86, AArch64 Instruction Set Architectures): -* throughput rate with target of zero packet loss ratio. -* at least one throughput rate with target of non-zero packet loss ratio. - - -In our experience, the higher the target loss ratio is, -the better is the repeatability of the corresponding load found. - -TODO: Define a good name for a load corresponding to a specific non-zero -target loss ration, while keeping Throughput for the load corresponding -to zero target loss ratio. - -This document RECOMMENDS the benchmark groups to search for corresponding loads -to at least one non-zero target loss ratio. -This document does not suggest any particular non-zero target loss ratio value -to search the corresponding load for. - -{::comment} - What is worse, some benchmark groups (which groups?; citation needed) - started reporting loads that achieved only "approximate zero loss", - while still calling that a Throughput (and thus becoming non-compliant - with RFC2544). -{:/comment} - -# Solution ideas - -This document gives several independent ideas on how to lower the (average) -overall search time, while remaining unconditionally compliant with RFC2544 -(and adding some of extensions). - -This document also specifies one particular way to combine all the ideas -into a single search algorithm class (single logic with few tweakable parameters). - -Little to no research has been done into the question of which combination -of ideas achieves the best compromise with respect to overal search time, -high repeatability and high comparability. - -TODO: How important it is to discuss particular implementation choices, -especially when motivated by non-deterministic SUT behavior? - -## Short duration trials - -https://datatracker.ietf.org/doc/html/rfc2544#section-24 -already mentions the possibity of using shorter duration -for trials that are not part of "final determination". - -Obviously, the upper and lower bound from a smaller duration trial -can be used as the initial upper and lower bound for the final determination. - -MLRsearch makes it clear a re-measurement is always needed -(new trial measurement with the same load but longer duration). -It also specifes what to do if the longer trial is no longer a valid bound -(TODO define?), e.g. start an external search. -Additionaly one halving can be saved during the shorter duration search. - -## FRMOL as reasonable start - -TODO expand: Overal search ends with "final determination" search, -preceded by "shorter duration search" preceded by "bound initialization", -where the bounds can be considerably different from min and max load. - -For SUTs with high repeatability, the FRMOL is usually a good approximation -of Throughput. But for less repeatable SUTs, forwarding rate (TODO define) -is frequently a bad approximation to Throughput, therefore halving -and other robust-to-worst-case approaches have to be used. -Still, forwarding rate at FRMOL load can be a good initial bound. - -## Non-zero loss ratios - -See the "Popularity of non-zero target loss ratios" section above. - -TODO: Define "trial measurement result classification criteria", -or keep reusing long phrases without definitions? - -A search for a load corresponding to a non-zero target loss rate -is very similar to a search for Throughput, -just the criterion when to increase or decrease the intended load -for the next trial measurement uses the comparison of trial loss ratio -to the target loss ratio (instead of comparing loss count to zero) -Any search algorithm that works for Throughput can be easily used also for -non-zero target loss rates, perhaps with small modifications -in places where the measured forwarding rate is used. - -Note that it is possible to search for multiple loss ratio goals if needed. - -## Concurrent ratio search - -A single trial measurement result can act as an upper bound for a lower -target loss ratio, and as a lower bound for a higher target loss ratio -at the same time. This is an example of how -it can be advantageous to search for all loss ratio goals "at once", -or at least "reuse" trial measurement result done so far. - -Even when a search algorithm is fully deterministic in load selection -while focusing on a single loss ratio and trial duration, -the choice of iteration order between target loss ratios and trial durations -can affect the obtained results in subtle ways. -MLRsearch offers one particular ordering. - -{::comment} - It is not clear if the current ordering is "best", - it is not even clear how to measure how good an ordering is. - We would need several models for bad SUT behaviors, - bug-free implementations of different orderings, - simulator to show the distribution of rates found, - distribution of overall durations, - and a criterion of which rate distribution is "bad" - and whether it is worth the time saved. -{:/comment} -{::comment} -{:/comment} - -## Load selection heuristics and shortcuts - -Aside of the two heuristics already mentioned (FRMOL based initial bounds -and saving one halving when increasing trial duration), -there are other tricks that can save some overall search time -at the cost of keeping the difference between final lower and upper bound -intentionally large (but still within the precision goal). - -TODO: Refer implementation subsections on: -* Uneven splits. -* Rounding the interval width up. -* Using old invalid bounds for interval width guessing. - -The impact on overall duration is probably small, -and the effect on result distribution maybe even smaller. -TODO: Is the two-liner above useful at all? - -# Non-compliance with RFC2544 - -It is possible to achieve even faster search times by abandoning -some requirements and suggestions of RFC2544, -mainly by reducing the wait times at start and end of trial. - -Such results are therefore no longer compliant with RFC2544 -(or at least not unconditionally), -but they may still be useful for internal usage, or for comparing -results of different DUTs achieved with an identical non-compliant algorithm. - -TODO: Refer to the subsection with CSIT customizations. - -# Additional Requirements - -RFC2544 can be understood as having a number of implicit requirements. -They are made explicit in this section -(as requirements for this document, not for RFC2544). - -Recommendations on how to properly address the implicit requirements -are out of scope of this document. - -{::comment} - - Although some (insufficient) ideas are proposed. - -{:/comment} - -## TODO: Search Stop Criteria - -TODO: Mention the timeout parameter? - -{::comment} - - TODO: highlight importance of results consistency - for SUT performance trending and anomaly detection. - -{:/comment} - -## Reliability of Test Equipment - -Both TG and TA MUST be able to handle correctly -every intended load used during the search. - -On TG side, the difference between Intended Load and Offered Load -MUST be small. - -TODO: How small? Difference of one packet may not be measurable -due to time uncertainties. - -{::comment} - - Maciek: 1 packet out of 10M, that's 10**-7 accuracy. - - Vratko: For example, TRex uses several "worker" threads, each doing its own - rounding on how many packets to send, separately per each traffic stream. - For high loads and durations, the observed number of frames transmitted - can differ from the expected (fractional) value by tens of frames. - -{:/comment} - -TODO expand: time uncertainty. - -To ensure that, max load (see Terminology) has to be set to low enough value. -Benchmark groups MAY list the max load value used, -especially if the Throughput value is equal (or close) to the max load. - -{::comment} - - The following is probably out of scope of this document, - but can be useful when put into a separate document. - - TODO expand: If it results in smaller Throughput reported, - it is not a big issue. Treat similarly to bandwidth and PPS limits of NICs. - - TODO expand: TA dropping packets when loaded only lowers Throughput, - so not an issue. - - TODO expand: TG sending less packets but stopping at target duration - is also fine, as long as the forwarding rate is used as Throughput value, - not the higher intended load. - - TODO expand: Duration stretching is not fine. - Neither "check for actual duration" nor "start+sleep+stop" - are reliable solutions due to time overheads and uncertainty - of TG starting/stopping traffic (and TA stopping counting packets). - -{:/comment} - -Solutions (even problem formulations) for the following open problems -are outside of the scope of this document: -* Detecting when the test equipment operates above its safe load. -* Finding a large but safe load value. -* Correcting any result affected by max load value not being a safe load. - -{::comment} - - TODO: Mention 90% of self-test as an idea: - https://datatracker.ietf.org/doc/html/rfc8219#section-9.2.1 - - This is pointing to DNS testing, nothing to do with throughput, - so how is it relevant here? - -{:/comment} - -{::comment} - - Part of discussion on BMWG mailing list (with small edits): - - This is a hard issue. - The algorithm as described has no way of knowing - which part of the whole system is limiting the performance. - - It could be SUT only (no problem, testing SUT as expected), - it could be TG only (can be mitigated by TG self-test - and using small enough loads). - - But it could also be an interaction between DUT and TG. - Imagine a TG (the Traffic Analyzer part) which is only able - to handle incoming traffic up to some rate, - but passes the self-test as the Generator part has maximal rate - not larger than that. But what if SUT turns that steady rate - into long-enough bursts of a higher rate (with delays between bursts - large enough, so average forwarding rate matches the load). - This way TA will see some packets as missing (when its buffers - fill up), even though SUT has processed them correctly. - -{:/comment} - -### Very late frames - -{::comment} - - In CSIT we are aggressive at skipping all wait times around trial, - but few of DUTs have large enough buffers. - Or there is another reason why we are seeing negative loss counts. - -{:/comment} - - -RFC2544 requires quite conservative time delays -see https://datatracker.ietf.org/doc/html/rfc2544#section-23 -to prevent frames buffered in one trial measurement -to be counted as received in a subsequent trial measurement. - -However, for some SUTs it may still be possible to buffer enough frames, -so they are still sending them (perhaps in bursts) -when the next trial measurement starts. -Sometimes, this can be detected as a negative trial loss count, e.g. TA receiving -more frames than TG has sent during this trial measurement. Frame duplication -is another way of causing the negative trial loss count. - -https://datatracker.ietf.org/doc/html/rfc2544#section-10 -recommends to use sequence numbers in frame payloads, -but generating and verifying them requires test equipment resources, -which may be not plenty enough to suport at high loads. -(Using low enough max load would work, but frequently that would be -smaller than SUT's sctual Throughput.) - -RFC2544 does not offer any solution to the negative loss problem, -except implicitly treating negative trial loss counts -the same way as positive trial loss counts. - -This document also does not offer any practical solution. - -Instead, this document SUGGESTS the search algorithm to take any precaution -necessary to avoid very late frames. - -This document also REQUIRES any detected duplicate frames to be counted -as additional lost frames. -This document also REQUIRES, any negative trial loss ratio -to be treated as positive trial loss ratio of the same absolute value. - -{::comment} - - !!! Make sure this is covered elsewere, at least in better comments. !!! - - ## TODO: Bad behavior of SUT - - (Highest load with always zero loss can be quite far from lowest load - with always nonzero loss.) - (Non-determinism: warm up, periodic "stalls", perf decrease over time, ...) - - Big buffers: - http://www.hit.bme.hu/~lencse/publications/ECC-2017-B-M-DNS64-revised.pdf - See page 8 and search for the word "gaming". - -{:/comment} - -!!! Nothing below is up-to-date with draft v02. !!! - -# MLRsearch Background - -TODO: Old section, probably obsoleted by preceding section(s). - -Multiple Loss Ratio search (MLRsearch) is a packet throughput search -algorithm suitable for deterministic systems (as opposed to -probabilistic systems). MLRsearch discovers multiple packet throughput -rates in a single search, each rate is associated with a distinct -Packet Loss Ratio (PLR) criterion. - -For cases when multiple rates need to be found, this property makes -MLRsearch more efficient in terms of time execution, compared to -traditional throughput search algorithms that discover a single packet -rate per defined search criteria (e.g. a binary search specified by -[RFC2544]). MLRsearch reduces execution time even further by relying on -shorter trial durations of intermediate steps, with only the final -measurements conducted at the specified final trial duration. This -results in the shorter overall search execution time when compared to a -traditional binary search, while guaranteeing the same results for -deterministic systems. - -In practice, two rates with distinct PLRs are commonly used for packet -throughput measurements of NFV systems: Non Drop Rate (NDR) with PLR=0 -and Partial Drop Rate (PDR) with PLR>0. The rest of this document -describes MLRsearch with NDR and PDR pair as an example. - -Similarly to other throughput search approaches like binary search, -MLRsearch is effective for SUTs/DUTs with PLR curve that is -non-decreasing with growing offered load. It may not be as -effective for SUTs/DUTs with abnormal PLR curves, although -it will always converge to some value. - -MLRsearch relies on traffic generator to qualify the received packet -stream as error-free, and invalidate the results if any disqualifying -errors are present e.g. out-of-sequence frames. - -MLRsearch can be applied to both uni-directional and bi-directional -throughput tests. - -For bi-directional tests, MLRsearch rates and ratios are aggregates of -both directions, based on the following assumptions: - -* Traffic transmitted by traffic generator and received by SUT/DUT - has the same packet rate in each direction, - in other words the offered load is symmetric. -* SUT/DUT packet processing capacity is the same in both directions, - resulting in the same packet loss under load. - -MLRsearch can be applied even without those assumptions, -but in that case the aggregate loss ratio is less useful as a metric. - -MLRsearch can be used for network transactions consisting of more than -just one packet, or anything else that has intended load as input -and loss ratio as output (duration as input is optional). -This text uses mostly packet-centric language. - -# MLRsearch Overview - -The main properties of MLRsearch: - -* MLRsearch is a duration aware multi-phase multi-rate search algorithm: - * Initial Phase determines promising starting interval for the search. - * Intermediate Phases progress towards defined final search criteria. - * Final Phase executes measurements according to the final search - criteria. - * Final search criteria are defined by following inputs: - * Target PLRs (e.g. 0.0 and 0.005 when searching for NDR and PDR). - * Final trial duration. - * Measurement resolution. -* Initial Phase: - * Measure MRR over initial trial duration. - * Measured MRR is used as an input to the first intermediate phase. -* Multiple Intermediate Phases: - * Trial duration: - * Start with initial trial duration in the first intermediate phase. - * Converge geometrically towards the final trial duration. - * Track all previous trial measurement results: - * Duration, offered load and loss ratio are tracked. - * Effective loss ratios are tracked. - * While in practice, real loss ratios can decrease with increasing load, - effective loss ratios never decrease. This is achieved by sorting - results by load, and using the effective loss ratio of the previous load - if the current loss ratio is smaller than that. - * The algorithm queries the results to find best lower and upper bounds. - * Effective loss ratios are always used. - * The phase ends if all target loss ratios have tight enough bounds. - * Search: - * Iterate over target loss ratios in increasing order. - * If both upper and lower bound are in measurement results for this duration, - apply bisect until the bounds are tight enough, - and continue with next loss ratio. - * If a bound is missing for this duration, but there exists a bound - from the previous duration (compatible with the other bound - at this duration), re-measure at the current duration. - * If a bound in one direction (upper or lower) is missing for this duration, - and the previous duration does not have a compatible bound, - compute the current "interval size" from the second tightest bound - in the other direction (lower or upper respectively) - for the current duration, and choose next offered load for external search. - * The logic guarantees that a measurement is never repeated with both - duration and offered load being the same. - * The logic guarantees that measurements for higher target loss ratio - iterations (still within the same phase duration) do not affect validity - and tightness of bounds for previous target loss ratio iterations - (at the same duration). - * Use of internal and external searches: - * External search: - * It is a variant of "exponential search". - * The "interval size" is multiplied by a configurable constant - (powers of two work well with the subsequent internal search). - * Internal search: - * A variant of binary search that measures at offered load between - the previously found bounds. - * The interval does not need to be split into exact halves, - if other split can get to the target width goal faster. - * The idea is to avoid returning interval narrower than the current - width goal. See sample implementation details, below. -* Final Phase: - * Executed with the final test trial duration, and the final width - goal that determines resolution of the overall search. -* Intermediate Phases together with the Final Phase are called - Non-Initial Phases. -* The returned bounds stay within prescribed min_rate and max_rate. - * When returning min_rate or max_rate, the returned bounds may be invalid. - * E.g. upper bound at max_rate may come from a measurement - with loss ratio still not higher than the target loss ratio. - -The main benefits of MLRsearch vs. binary search include: - -* In general, MLRsearch is likely to execute more trials overall, but - likely less trials at a set final trial duration. -* In well behaving cases, e.g. when results do not depend on trial - duration, it greatly reduces (>50%) the overall duration compared to a - single PDR (or NDR) binary search over duration, while finding - multiple drop rates. -* In all cases MLRsearch yields the same or similar results to binary - search. -* Note: both binary search and MLRsearch are susceptible to reporting - non-repeatable results across multiple runs for very bad behaving - cases. - -Caveats: - -* Worst case MLRsearch can take longer than a binary search, e.g. in case of - drastic changes in behaviour for trials at varying durations. - * Re-measurement at higher duration can trigger a long external search. - That never happens in binary search, which uses the final duration - from the start. - -# Sample Implementation - -Following is a brief description of a sample MLRsearch implementation, -which is a simplified version of the existing implementation. - -## Input Parameters - -1. **max_rate** - Maximum Transmit Rate (MTR) of packets to - be used by external traffic generator implementing MLRsearch, - limited by the actual Ethernet link(s) rate, NIC model or traffic - generator capabilities. -2. **min_rate** - minimum packet transmit rate to be used for - measurements. MLRsearch fails if lower transmit rate needs to be - used to meet search criteria. -3. **final_trial_duration** - required trial duration for final rate - measurements. -4. **initial_trial_duration** - trial duration for initial MLRsearch phase. -5. **final_relative_width** - required measurement resolution expressed as - (lower_bound, upper_bound) interval width relative to upper_bound. -6. **packet_loss_ratios** - list of maximum acceptable PLR search criteria. -7. **number_of_intermediate_phases** - number of phases between the initial - phase and the final phase. Impacts the overall MLRsearch duration. - Less phases are required for well behaving cases, more phases - may be needed to reduce the overall search duration for worse behaving cases. - -## Initial Phase - -1. First trial measures at configured maximum transmit rate (MTR) and - discovers maximum receive rate (MRR). - * IN: trial_duration = initial_trial_duration. - * IN: offered_transmit_rate = maximum_transmit_rate. - * DO: single trial. - * OUT: measured loss ratio. - * OUT: MRR = measured receive rate. - Received rate is computed as intended load multiplied by pass ratio - (which is one minus loss ratio). This is useful when loss ratio is computed - from a different metric than intended load. For example, intended load - can be in transactions (multiple packets each), but loss ratio is computed - on level of packets, not transactions. - - * Example: If MTR is 10 transactions per second, and each transaction has - 10 packets, and receive rate is 90 packets per second, then loss rate - is 10%, and MRR is computed to be 9 transactions per second. - - If MRR is too close to MTR, MRR is set below MTR so that interval width - is equal to the width goal of the first intermediate phase. - If MRR is less than min_rate, min_rate is used. -2. Second trial measures at MRR and discovers MRR2. - * IN: trial_duration = initial_trial_duration. - * IN: offered_transmit_rate = MRR. - * DO: single trial. - * OUT: measured loss ratio. - * OUT: MRR2 = measured receive rate. - If MRR2 is less than min_rate, min_rate is used. - If loss ratio is less or equal to the smallest target loss ratio, - MRR2 is set to a value above MRR, so that interval width is equal - to the width goal of the first intermediate phase. - MRR2 could end up being equal to MTR (for example if both measurements so far - had zero loss), which was already measured, step 3 is skipped in that case. -3. Third trial measures at MRR2. - * IN: trial_duration = initial_trial_duration. - * IN: offered_transmit_rate = MRR2. - * DO: single trial. - * OUT: measured loss ratio. - * OUT: MRR3 = measured receive rate. - If MRR3 is less than min_rate, min_rate is used. - If step 3 is not skipped, the first trial measurement is forgotten. - This is done because in practice (if MRR2 is above MRR), external search - from MRR and MRR2 is likely to lead to a faster intermediate phase - than a bisect between MRR2 and MTR. - -## Non-Initial Phases - -1. Main phase loop: - 1. IN: trial_duration for the current phase. Set to - initial_trial_duration for the first intermediate phase; to - final_trial_duration for the final phase; or to the element of - interpolating geometric sequence for other intermediate phases. - For example with two intermediate phases, trial_duration of the - second intermediate phase is the geometric average of - initial_trial_duration and final_trial_duration. - 2. IN: relative_width_goal for the current phase. Set to - final_relative_width for the final phase; doubled for each - preceding phase. For example with two intermediate phases, the - first intermediate phase uses quadruple of final_relative_width - and the second intermediate phase uses double of - final_relative_width. - 3. IN: Measurement results from the previous phase (previous duration). - 4. Internal target ratio loop: - 1. IN: Target loss ratio for this iteration of ratio loop. - 2. IN: Measurement results from all previous ratio loop iterations - of current phase (current duration). - 3. DO: According to the procedure described in point 2: - 1. either exit the phase (by jumping to 1.5), - 2. or exit loop iteration (by continuing with next target loss ratio, - jumping to 1.4.1), - 3. or calculate new transmit rate to measure with. - 4. DO: Perform the trial measurement at the new transmit rate and - current trial duration, compute its loss ratio. - 5. DO: Add the result and go to next iteration (1.4.1), - including the added trial result in 1.4.2. - 5. OUT: Measurement results from this phase. - 6. OUT: In the final phase, bounds for each target loss ratio - are extracted and returned. - 1. If a valid bound does not exist, use min_rate or max_rate. -2. New transmit rate (or exit) calculation (for point 1.4.3): - 1. If the previous duration has the best upper and lower bound, - select the middle point as the new transmit rate. - 1. See 2.5.3. below for the exact splitting logic. - 2. This can be a no-op if interval is narrow enough already, - in that case continue with 2.2. - 3. Discussion, assuming the middle point is selected and measured: - 1. Regardless of loss rate measured, the result becomes - either best upper or best lower bound at current duration. - 2. So this condition is satisfied at most once per iteration. - 3. This also explains why previous phase has double width goal: - 1. We avoid one more bisection at previous phase. - 2. At most one bound (per iteration) is re-measured - with current duration. - 3. Each re-measurement can trigger an external search. - 4. Such surprising external searches are the main hurdle - in achieving low overall search durations. - 5. Even without 1.1, there is at most one external search - per phase and target loss ratio. - 6. But without 1.1 there can be two re-measurements, - each coming with a risk of triggering external search. - 2. If the previous duration has one bound best, select its transmit rate. - In deterministic case this is the last measurement needed this iteration. - 3. If only upper bound exists in current duration results: - 1. This can only happen for the smallest target loss ratio. - 2. If the upper bound was measured at min_rate, - exit the whole phase early (not investigating other target loss ratios). - 3. Select new transmit rate using external search: - 1. For computing previous interval size, use: - 1. second tightest bound at current duration, - 2. or tightest bound of previous duration, - if compatible and giving a more narrow interval, - 3. or target interval width if none of the above is available. - 4. In any case increase to target interval width if smaller. - 2. Quadruple the interval width. - 3. Use min_rate if the new transmit rate is lower. - 4. If only lower bound exists in current duration results: - 1. If the lower bound was measured at max_rate, - exit this iteration (continue with next lowest target loss ratio). - 2. Select new transmit rate using external search: - 1. For computing previous interval size, use: - 1. second tightest bound at current duration, - 2. or tightest bound of previous duration, - if compatible and giving a more narrow interval, - 3. or target interval width if none of the above is available. - 4. In any case increase to target interval width if smaller. - 2. Quadruple the interval width. - 3. Use max_rate if the new transmit rate is higher. - 5. The only remaining option is both bounds in current duration results. - 1. This can happen in two ways, depending on how the lower bound - was chosen. - 1. It could have been selected for the current loss ratio, - e.g. in re-measurement (2.2) or in initial bisect (2.1). - 2. It could have been found as an upper bound for the previous smaller - target loss ratio, in which case it might be too low. - 3. The algorithm does not track which one is the case, - as the decision logic works well regardless. - 2. Compute "extending down" candidate transmit rate exactly as in 2.3. - 3. Compute "bisecting" candidate transmit rate: - 1. Compute the current interval width from the two bounds. - 2. Express the width as a (float) multiple of the target width goal - for this phase. - 3. If the multiple is not higher than one, it means the width goal - is met. Exit this iteration and continue with next higher - target loss ratio. - 4. If the multiple is two or less, use half of that - for new width if the lower subinterval. - 5. Round the multiple up to nearest even integer. - 6. Use half of that for new width if the lower subinterval. - 7. Example: If lower bound is 2.0 and upper bound is 5.0, and width - goal is 1.0, the new candidate transmit rate will be 4.0. - This can save a measurement when 4.0 has small loss. - Selecting the average (3.5) would never save a measurement, - giving more narrow bounds instead. - 4. If either candidate computation want to exit the iteration, - do as bisecting candidate computation says. - 5. The remaining case is both candidates wanting to measure at some rate. - Use the higher rate. This prefers external search down narrow enough - interval, competing with perfectly sized lower bisect subinterval. - -# FD.io CSIT Implementation - -The only known working implementation of MLRsearch is in -the open-source code running in Linux Foundation -FD.io CSIT project [FDio-CSIT-MLRsearch] as part of -a Continuous Integration / Continuous Development (CI/CD) framework. - -MLRsearch is also available as a Python package in [PyPI-MLRsearch]. - -## Additional details - -This document so far has been describing a simplified version of -MLRsearch algorithm. The full algorithm as implemented in CSIT contains -additional logic, which makes some of the details (but not general -ideas) above incorrect. Here is a short description of the additional -logic as a list of principles, explaining their main differences from -(or additions to) the simplified description, but without detailing -their mutual interaction. - -1. Logarithmic transmit rate. - * In order to better fit the relative width goal, the interval - doubling and halving is done differently. - * For example, the middle of 2 and 8 is 4, not 5. -2. Timeout for bad cases. - * The worst case for MLRsearch is when each phase converges to - intervals way different than the results of the previous phase. - * Rather than suffer total search time several times larger than pure - binary search, the implemented tests fail themselves when the - search takes too long (given by argument *timeout*). -3. Intended count. - * The number of packets to send during the trial should be equal to - the intended load multiplied by the duration. - * Also multiplied by a coefficient, if loss ratio is calculated - from a different metric. - * Example: If a successful transaction uses 10 packets, - load is given in transactions per second, but loss ratio is calculated - from packets, so the coefficient to get intended count of packets - is 10. - * But in practice that does not work. - * It could result in a fractional number of packets, - * so it has to be rounded in a way traffic generator chooses, - * which may depend on the number of traffic flows - and traffic generator worker threads. -4. Attempted count. As the real number of intended packets is not known exactly, - the computation uses the number of packets traffic generator reports as sent. - Unless overridden by the next point. -5. Duration stretching. - * In some cases, traffic generator may get overloaded, - causing it to take significantly longer (than duration) to send all packets. - * The implementation uses an explicit stop, - * causing lower attempted count in those cases. - * The implementation tolerates some small difference between - attempted count and intended count. - * 10 microseconds worth of traffic is sufficient for our tests. - * If the difference is higher, the unsent packets are counted as lost. - * This forces the search to avoid the regions of high duration stretching. - * The final bounds describe the performance of not just SUT, - but of the whole system, including the traffic generator. -6. Excess packets. - * In some test (e.g. using TCP flows) Traffic generator reacts to packet loss - by retransmission. Usually, such packet loss is already affecting loss ratio. - If a test also wants to treat retransmissions due to heavily delayed packets - also as a failure, this is once again visible as a mismatch between - the intended count and the attempted count. - * The CSIT implementation simply looks at absolute value of the difference, - so it offers the same small tolerance before it starts marking a "loss". -7. For result processing, we use lower bounds and ignore upper bounds. - -### FD.io CSIT Input Parameters - -1. **max_rate** - Typical values: 2 * 14.88 Mpps for 64B - 10GE link rate, 2 * 18.75 Mpps for 64B 40GE NIC (specific model). -2. **min_rate** - Value: 2 * 9001 pps (we reserve 9000 pps - for latency measurements). -3. **final_trial_duration** - Value: 30.0 seconds. -4. **initial_trial_duration** - Value: 1.0 second. -5. **final_relative_width** - Value: 0.005 (0.5%). -6. **packet_loss_ratios** - Value: 0.0, 0.005 (0.0% for NDR, 0.5% for PDR). -7. **number_of_intermediate_phases** - Value: 2. - The value has been chosen based on limited experimentation to date. - More experimentation needed to arrive to clearer guidelines. -8. **timeout** - Limit for the overall search duration (for one search). - If MLRsearch oversteps this limit, it immediately declares the test failed, - to avoid wasting even more time on a misbehaving SUT. - Value: 600.0 (seconds). -9. **expansion_coefficient** - Width multiplier for external search. - Value: 4.0 (interval width is quadroupled). - Value of 2.0 is best for well-behaved SUTs, but value of 4.0 has been found - to decrease overall search time for worse-behaved SUT configurations, - contributing more to the overall set of different SUT configurations tested. - - -## Example MLRsearch Run - - -The following list describes a search from a real test run in CSIT -(using the default input values as above). - -* Initial phase, trial duration 1.0 second. - -Measurement 1, intended load 18750000.0 pps (MTR), -measured loss ratio 0.7089514628479618 (valid upper bound for both NDR and PDR). - -Measurement 2, intended load 5457160.071600716 pps (MRR), -measured loss ratio 0.018650817320118702 (new tightest upper bounds). - -Measurement 3, intended load 5348832.933500009 pps (slightly less than MRR2 -in preparation for first intermediate phase target interval width), -measured loss ratio 0.00964383362905351 (new tightest upper bounds). - -* First intermediate phase starts, trial duration still 1.0 seconds. - -Measurement 4, intended load 4936605.579021453 pps (no lower bound, -performing external search downwards, for NDR), -measured loss ratio 0.0 (valid lower bound for both NDR and PDR). - -Measurement 5, intended load 5138587.208637197 pps (bisecting for NDR), -measured loss ratio 0.0 (new tightest lower bounds). - -Measurement 6, intended load 5242656.244044665 pps (bisecting), -measured loss ratio 0.013523745379347257 (new tightest upper bounds). - -* Both intervals are narrow enough. -* Second intermediate phase starts, trial duration 5.477225575051661 seconds. - -Measurement 7, intended load 5190360.904111567 pps (initial bisect for NDR), -measured loss ratio 0.0023533920869969953 (NDR upper bound, PDR lower bound). - -Measurement 8, intended load 5138587.208637197 pps (re-measuring NDR lower bound), -measured loss ratio 1.2080222912800403e-06 (new tightest NDR upper bound). - -* The two intervals have separate bounds from now on. - -Measurement 9, intended load 4936605.381062318 pps (external NDR search down), -measured loss ratio 0.0 (new valid NDR lower bound). - -Measurement 10, intended load 5036583.888432355 pps (NDR bisect), -measured loss ratio 0.0 (new tightest NDR lower bound). - -Measurement 11, intended load 5087329.903232804 pps (NDR bisect), -measured loss ratio 0.0 (new tightest NDR lower bound). - -* NDR interval is narrow enough, PDR interval not ready yet. - -Measurement 12, intended load 5242656.244044665 pps (re-measuring PDR upper bound), -measured loss ratio 0.0101174866190136 (still valid PDR upper bound). - -* Also PDR interval is narrow enough, with valid bounds for this duration. -* Final phase starts, trial duration 30.0 seconds. - -Measurement 13, intended load 5112894.3238511775 pps (initial bisect for NDR), -measured loss ratio 0.0 (new tightest NDR lower bound). - -Measurement 14, intended load 5138587.208637197 (re-measuring NDR upper bound), -measured loss ratio 2.030389804256833e-06 (still valid PDR upper bound). - -* NDR interval is narrow enough, PDR interval not yet. - -Measurement 15, intended load 5216443.04126728 pps (initial bisect for PDR), -measured loss ratio 0.005620871287975237 (new tightest PDR upper bound). - -Measurement 16, intended load 5190360.904111567 (re-measuring PDR lower bound), -measured loss ratio 0.0027629971184465604 (still valid PDR lower bound). - -* PDR interval is also narrow enough. -* Returning bounds: -* NDR_LOWER = 5112894.3238511775 pps; NDR_UPPER = 5138587.208637197 pps; -* PDR_LOWER = 5190360.904111567 pps; PDR_UPPER = 5216443.04126728 pps. - -# IANA Considerations - -No requests of IANA. - -# Security Considerations - -Benchmarking activities as described in this memo are limited to -technology characterization of a DUT/SUT using controlled stimuli in a -laboratory environment, with dedicated address space and the constraints -specified in the sections above. - -The benchmarking network topology will be an independent test setup and -MUST NOT be connected to devices that may forward the test traffic into -a production network or misroute traffic to the test management network. - -Further, benchmarking is performed on a "black-box" basis, relying -solely on measurements observable external to the DUT/SUT. - -Special capabilities SHOULD NOT exist in the DUT/SUT specifically for -benchmarking purposes. Any implications for network security arising -from the DUT/SUT SHOULD be identical in the lab and in production -networks. - -# Acknowledgements - -Many thanks to Alec Hothan of OPNFV NFVbench project for thorough -review and numerous useful comments and suggestions. - ---- back diff --git a/docs/ietf/draft-ietf-bmwg-mlrsearch-03.md b/docs/ietf/draft-ietf-bmwg-mlrsearch-03.md new file mode 100644 index 0000000000..40180dc55b --- /dev/null +++ b/docs/ietf/draft-ietf-bmwg-mlrsearch-03.md @@ -0,0 +1,501 @@ +--- +title: Multiple Loss Ratio Search +abbrev: MLRsearch +docname: draft-ietf-bmwg-mlrsearch-03 +date: 2022-11-09 + +ipr: trust200902 +area: ops +wg: Benchmarking Working Group +kw: Internet-Draft +cat: info + +coding: us-ascii +pi: # can use array (if all yes) or hash here + toc: yes + sortrefs: # defaults to yes + symrefs: yes + +author: + - + ins: M. Konstantynowicz + name: Maciek Konstantynowicz + org: Cisco Systems + email: mkonstan@cisco.com + - + ins: V. Polak + name: Vratko Polak + org: Cisco Systems + email: vrpolak@cisco.com + +normative: + RFC1242: + RFC2285: + RFC2544: + RFC9004: + +informative: + TST009: + target: https://www.etsi.org/deliver/etsi_gs/NFV-TST/001_099/009/03.04.01_60/gs_NFV-TST009v030401p.pdf + title: "TST 009" + FDio-CSIT-MLRsearch: + target: https://s3-docs.fd.io/csit/rls2110/report/introduction/methodology_data_plane_throughput/methodology_data_plane_throughput.html#mlrsearch-tests + title: "FD.io CSIT Test Methodology - MLRsearch" + date: 2021-11 + PyPI-MLRsearch: + target: https://pypi.org/project/MLRsearch/0.3.0/ + title: "MLRsearch 0.3.0, Python Package Index" + date: 2021-04 + +--- abstract + +This document proposes improvements to [RFC2544] throughput search by +defining a new methodology called Multiple Loss Ratio search +(MLRsearch). The main objectives for MLRsearch are to minimize the +total test duration, search for multiple loss ratios and improve +results repeatibility and comparability. + +The main motivation behind MLRsearch is the new set of challenges and +requirements posed by testing Network Function Virtualization +(NFV) systems and other software based network data planes. + +MLRsearch offers several ways to address these challenges, giving user +configuration options to select their way. + +--- middle + +{::comment} + As we use kramdown to convert from markdown, + we use this way of marking comments not to be visible in rendered draft. + https://stackoverflow.com/a/42323390 + If other engine is used, convert to this way: + https://stackoverflow.com/a/20885980 +{:/comment} + +# Purpose and Scope + +The purpose of this document is to describe Multiple Loss Ratio search +(MLRsearch), a throughput search methodology optimized for software +DUTs. + +Applying vanilla [RFC2544] throughput bisection to software DUTs +results in a number of problems: + +- Binary search takes too long as most of trials are done far from the + eventually found throughput. +- The required final trial duration and pauses between trials also + prolong the overall search duration. +- Software DUTs show noisy trial results (noisy neighbor problem), + leading to big spread of possible discovered throughput values. +- Throughput requires loss of exactly zero packets, but the industry + frequently allows for small but non-zero losses. +- The definition of throughput is not clear when trial results are + inconsistent. + +MLRsearch aims to address these problems by applying the following set +of enhancements: + +- Allow searching with multiple loss ratio goals. + - Each trial result can affect any search goal in principle + (trial reuse). +- Multiple phases within one loss ratio goal search, middle ones need + to spend less time on trials. + - Middle phases also aim at lesser precision. + - Use Forwarding Rate (FR) at maximum offered load + [RFC2285] (section 3.6.2) to initialize the first middle phase. +- Take care when dealing with inconsistent trial results. + - Loss ratios goals are handled in an order that precludes any + interference from later trials to earlier goals. +- Apply several load selection heuristics to save even more time + by trying hard to avoid unnecessarily narrow intervals. + +MLRsearch configuration options are flexible enough to +support both conservative settings (unconditionally compliant with [RFC2544], +but longer search duration and worse repeatability) and aggressive +settings (shorter search duration and better repeatability but not +compliant with [RFC2544]). + +No part of [RFC2544] is intended to be obsoleted by this document. + +# Problems + +## Long Test Duration + +Emergence of software DUTs, with frequent software updates and a +number of different packet processing modes and configurations, drives +the requirement of continuous test execution and bringing down the test +execution time. + +In the context of characterising particular DUT's network performance, this +calls for improving the time efficiency of throughput search. +A vanilla bisection (at 60sec trial duration for unconditional [RFC2544] +compliance) is slow, because most trials spend time quite far from the +eventual throughput. + +[RFC2544] does not specify any stopping condition for throughput search, +so users can trade-off between search duration and precision goal. +But, due to exponential behavior of bisection, small improvement +in search duration needs relatively big sacrifice in the result precision. + +## DUT within SUT + +[RFC2285] defines: +- *DUT* as + - The network forwarding device to which stimulus is offered and + response measured [RFC2285] (section 3.1.1). +- *SUT* as + - The collective set of network devices to which stimulus is offered + as a single entity and response measured [RFC2285] (section 3.1.2). + +[RFC2544] specifies a test setup with an external tester stimulating the +networking system, treating it either as a single DUT, or as a system +of devices, an SUT. + +In case of software networking, the SUT consists of a software program +processing packets (device of interest, the DUT), +running on a server hardware and using operating system functions as appropriate, +with server hardware resources shared across all programs +and the operating system. + +DUT is effectively "nested" within SUT. + +Due to a shared multi-tenant nature of SUT, DUT is subject to +interference (noise) coming from the operating system and any other +software running on the same server. Some sources of noise can be +eliminated (e.g. by pinning DUT program threads to specific CPU cores +and isolating those cores to avoid context switching). But some +noise remains after all such reasonable precautions are applied. This +noise does negatively affect DUT's network performance. We refer to it +as an *SUT noise*. + +DUT can also exhibit fluctuating performance itself, e.g. while performing +some "stop the world" internal stateful processing. In many cases this +may be an expected per-design behavior, as it would be observable even +in a hypothetical scenario where all sources of SUT noise are +eliminated. Such behavior affects trial results in a way similar to SUT +noise. We use *noise* as a shorthand covering both *DUT fluctuations* and +genuine SUT noise. + +A simple model of SUT performance consists of a baseline *noiseless performance*, +and an additional noise. The baseline is assumed to be constant (enough). +The noise varies in time, sometimes wildly. The noise can sometimes be negligible, +but frequently it lowers the observed SUT performance in a trial. + +In this model, SUT does not have a single performance value, it has a spectrum. +One end of the spectrum is the noiseless baseline, +the other end is a *noiseful performance*. In practice, trial results +close to the noiseful end of the spectrum happen only rarely. +The worse performance, the more rarely it is seen. + +Focusing on DUT, the benchmarking effort should aim +at eliminating only the SUT noise from SUT measurement. +But that is not really possible, as there are no realistic enough models +able to distinguish SUT noise from DUT fluctuations. + +However, assuming that a well-constructed SUT has the DUT as its +performance bottleneck, the "DUT noiseless performance" can be defined +as the noiseless end of SUT performance spectrum. (At least for +throughput. For other quantities such as latency there will be an +additive difference.) By this definition, DUT noiseless performance +also minimizes the impact of DUT fluctuations. + +In this document, we reduce the "DUT within SUT" problem to estimating +the noiseless end of SUT performance spectrum from a limited number of +trial results. + +Any improvements to throughput search algorithm, aimed for better +dealing with software networking SUT and DUT setup, should employ +strategies recognizing the presence of SUT noise, and allow discovery of +(proxies for) DUT noiseless performance +at different levels of sensitivity to SUT noise. + +## Repeatability and Comparability + +[RFC2544] does not suggest to repeat throughput search, and from just one +throughput value, it cannot be determined how repeatable that value is. +In practice, poor repeatability is also the main cause of poor +comparability, e.g. different benchmarking teams can test the same DUT +but get different throughput values. + +[RFC2544] throughput requirements (60s trial, no tolerance to single frame loss) +force the search to converge around the noiseful end of SUT performance +spectrum. As that end is affected by rare trials of significantly low +performance, the resulting throughput repeatability is poor. + +The repeatability problem is the problem of defining a search procedure +which reports more stable results +(even if they can no longer be called "throughput" in [RFC2544] sense). +According to baseline (noiseless) and noiseful model, better repeatability +will be at the noiseless end of the spectrum. +Therefore, solutions to the "DUT within SUT" problem +will help also with the repeatability problem. + +Conversely, any alteration to [RFC2544] throughput search +that improves repeatability should be considered +as less dependent on the SUT noise. + +An alternative option is to simply run a search multiple times, and report some +statistics (e.g. average and standard deviation). This can be used +for "important" tests, but it makes the search duration problem even +bigger. + +## Throughput with Non-Zero Loss + +[RFC1242] (section 3.17) defines throughput as: + The maximum rate at which none of the offered frames + are dropped by the device. + +and then it says: + Since even the loss of one frame in a + data stream can cause significant delays while + waiting for the higher level protocols to time out, + it is useful to know the actual maximum data + rate that the device can support. + +Contrary to that, many benchmarking teams settle with non-zero +(small) loss ratio as the goal for a "throughput rate". + +Motivations are many: modern protocols tolerate frame loss better; +trials nowadays send way more frames within the same duration; +impact of rare noise bursts is smaller as the baseline performance +can compensate somewhat by keeping the loss ratio below the goal; +if SUT noise with "ideal DUT" is known, it can be set as the loss ratio goal. + +Regardless of validity of any and all similar motivations, +support for non-zero loss goals makes any search algorithm more user-friendly. +[RFC2544] throughput is not friendly in this regard. + +Searching for multiple loss ratio goals also helps to describe the SUT +performance better than a single goal result. Repeated wide gap between +zero and non-zero loss loads indicates the noise has a large impact on +the overall SUT performance. + +It is easy to modify the vanilla bisection to find a lower bound +for intended load that satisfies a non-zero-loss goal, +but it is not that obvious how to search for multiple goals at once, +hence the support for multiple loss goals remains a problem. + +## Inconsistent Trial Results + +While performing throughput search by executing a sequence of +measurement trials, there is a risk of encountering inconsistencies +between trial results. + +The plain bisection never encounters inconsistent trials. +But [RFC2544] hints about possibility if inconsistent trial results in two places. +The first place is section 24 where full trial durations are required, presumably +because they can be inconsistent with results from shorter trial durations. +The second place is section 26.3 where two successive zero-loss trials +are recommended, presumably because after one zero-loss trial +there can be subsequent inconsistent non-zero-loss trial. + +Examples include: + +- a trial at the same load (same or different trial duration) results + in a different packet loss ratio. +- a trial at higher load (same or different trial duration) results + in a smaller packet loss ratio. + +Any robust throughput search algorithm needs to decide how to continue +the search in presence of such inconsistencies. +Definitions of throughput in [RFC1242] and [RFC2544] are not specific enough +to imply a unique way of handling such inconsistencies. + +Ideally, there will be a definition of a quantity which both generalizes +throughput for non-zero-loss (and other possible repeatibility enhancements), +while being precise enough to force a specific way to resolve trial +inconsistencies. +But until such definition is agreed upon, the correct way to handle +inconsistent trial results remains an open problem. + +# MLRsearch Approach + +The following description intentionally leaves out some important implementation +details. This is both to hide complexity that is not important for overall +understanding, and to allow future improvements in the implementation. + +## Terminology + +- *trial duration*: Amount of time over which frames are transmitted + towards SUT and DUT in a single measurement step. + - **MLRsearch input parameter** for final MLRsearch measurements. +- *loss ratio*: Ratio of the count of frames lost to the count of frames + transmitted over a trial duration, a.k.a. packet loss ratio. Related + to packet loss rate [RFC1242] (section 3.6). + In MLRsearch loss ratio can mean either a trial result or a goal: + - *trial loss ratio*: Loss ratio measured during a trial. + - *loss ratio goal*: **MLRsearch input parameter**. + - If *trial loss ratio* is smaller or equal to this, + the trial **satisfies** the loss ratio goal. +- *load*: Constant offered load stimulating the SUT and DUT. Consistent + with offered load [RFC2285] (section 3.5.2). + - MLRsearch works with intended load instead, as it cannot deal with + situations where the offered load is considerably different than + intended load. +- *throughput*: The maximum load at which none of the offered frames are + dropped by the SUT and DUT. Consistent with [RFC1242] (section 3.17). +- *conditional throughput*: The forwarding rate measured at the maximum + load at which a list of specified conditions are met i.e. loss ratio + goal and trial duration. + - Throughput is then a special case of conditional throughput + for zero loss ratio goal and long enough trial duration. + - Conditional throughput is aligned with forwarding rate (FR) + [RFC2285] (section 3.6.1), adding trial duration to offered load + required when reporting FR. +- *lower bound*: One of values tracked by MLRsearch during the search runtime. + It is specific to the current trial duration and current loss ratio goal. + It represents a load value with at least one trial result available. + If the trial satisfies the current loss ratio goal, + it is a *valid* bound (else *invalid*). +- *upper bound*: One of values tracked by MLRsearch during the search runtime. + It is specific to the current trial duration and current loss ratio goal. + It represents a load value with at least one trial result available. + If the trial satisfies the current loss ratio goal, + it is an *invalid* bound (else *valid*). +- *interval*: The span between lower and upper bound loads. +- *precision goal*: **MLRsearch input parameter**, acting as a search + stop condition, given as either absolute or relative width goal. An + interval meets precision goal if: + - The difference of upper and lower bound loads (in pps) + is not more than the absolute width goal. + - The difference as above, divided by upper bound load (in pps) + is not more than the relative width goal. + +## Description + +The MLRsearch approach to address the identified problems is based +on the following main strategies: + +- MLRsearch main inputs include the following search goals and parameters: + - One or more **loss ratio goals**. + - e.g. a zero-loss goal and one (or more) non-zero-loss goals. + - **Target trial duration** condition governing required trial duration + for final measurements. + - **Target precision** condition governing how close final lower and + upper bound load values must be to each other for final + measurements. +- Search is executed as a sequence of phases: + - *Initial phase* initializes bounds for the first middle phase. + - *Middle phase*s narrow down the bounds, using shorter trial + durations and lower precision goals. Several middle phases can + precede each final phase. + - *Final phase* (one per loss ratio goal) finds bounds matching input + goals and parameters to serve as the overal search output. +- Each search phase produces its *ending* upper bound and lower bound: + - Initial phase may produce invalid bounds. + - Middle and final phases produce valid bounds. + - Middle or final phases needs at least two values to act as + *starting* bounds (may be invalid). + - Each phase may perform several trial measurements, until phase's + ending conditions are all met. + - Trial results from previous phases may be re-used. +- Initial phase establishes the starting values for bounds, using + forwarding rates (FR) [RFC2285] (section 3.6.1) + from a few trials of minimal duration, as follows: + - 1st trial is done at *maximum offered load (MOL)* [RFC2285] (section 3.5.3), + resulting in Forwarding rate at maximum offered load (FRMOL) + [RFC2285] (section 3.6.2). + - 2nd trial is done at *FRMOL*, resulting in forwarding rate at FRMOL (FRFRMOL), + newly defined here. + - 3rd trial is done at *FRFRMOL*, so its results are available for the next phase. + - By default, FRMOL is used as an upper bound, FRFRMOL as a lower bound. + - Adjustments may apply here for some cases e.g. when 2nd trial got + zero loss or if FRFRMOL is too close to FRMOL. +- Middle phases are producing ending bounds by improving upon starting bounds: + - Each middle phase uses the same loss ratio goal as the final phase it precedes. + - Called *current loss ratio goal* for upper and lower bound purposes. + - Each middle phase has its own *current trial duration* + and *current precision goal* parameters, computed from + MLRsearch input parameters. + As phases progress, these parameters approach MLRsearch main input values. + - Current trial duration starts from a configurable minimum (e.g. 1 sec) + and increases in a geometric sequence. + - Current precision goal always allows twice as wide intervals + as the following phase. + - The starting bounds are usually the ending bounds from the preceding phase. + - Unless there are many previous trial results that are more promising. + - Each middle phase operates in a sequence of four actions: + 1. Perform trial at the load between the starting bounds. + - Depending on the trial result this becomes the first + new valid upper or lower bound for current phase. + 2. Re-measure at the remaining starting lower or upper (respectively) bound. + 3. If that did not result in a valid bound, start an *external search*. + - That is a variant of exponential search. + - The "growth" is given by input parameter *expansion_coefficient*. + - This action ends when a new valid bound is found. + - Or if an already existing valid bound becomes close enough. + 4. Repeatedly bisect the current interval until the bounds are close enough. +- Final search phase operates in exactly the same way as middle phases. + There are two reasons why it is named differently: + - The current trial duration and current precision goal within the phase + are equal to the target trial duration and target precision input parameters. + - The forwarding rates of the ending bounds become the output of MLRsearch. + - Specifically, the forwarding rates of the final lower bounds + are the conditional throughput values per given loss ratio goals. + +## Enhancement: Multiple trials per load + +An enhancement of MLRsearch is to introduce a *noise tolerance* input parameter. +The idea is to perform several medium-length trials (instead of a single long trial) +and tolerate a configurable fraction of them to not-satisfy the loss ratio goal. + +MLRsearch implementation with this enhancement exists in FD.io CSIT project +and test results of VPP and DPDK (testpmd, l3fwd) DUTs look promising. + +This enhancement would make the description of MLRsearch approach +considerably more complicated, so this document version only describes +MLRsearch without this enhancement. + +# How the problems are addressed + +Configurable loss ratio goals are in direct support for non-zero-loss conditional througput. +In practice the conditional throughput results' stability +increases with higher loss ratio goals. + +Multiple trials with noise tolerance enhancement will also indirectly +increase result stability and it will allow MLRsearch +to add all the benefits of Binary Search with Loss Verification, +as recommended in [RFC9004] (section 6.2) +and specified in [TST009] (section 12.3.3). + +The main factor improving the overall search time is the introduction +of middle phases. The full implementation can bring a large number of +heuristics related to how exactly should the next trial load be chosen, +but the impact of those is not as big. + +The Description subsection lacks any details on how to handle inconsistent +trial results. In practice, there tend to be a three-way trade-off +between i) short overall search time, ii) result stability +and iii) how simple the definition of the returned conditional throughput can be. +The third one is important for comparability between different MLRsearch +implementations. + +# IANA Considerations + +No requests of IANA. + +# Security Considerations + +Benchmarking activities as described in this memo are limited to +technology characterization of a DUT/SUT using controlled stimuli in a +laboratory environment, with dedicated address space and the constraints +specified in the sections above. + +The benchmarking network topology will be an independent test setup and +MUST NOT be connected to devices that may forward the test traffic into +a production network or misroute traffic to the test management network. + +Further, benchmarking is performed on a "black-box" basis, relying +solely on measurements observable external to the DUT/SUT. + +Special capabilities SHOULD NOT exist in the DUT/SUT specifically for +benchmarking purposes. Any implications for network security arising +from the DUT/SUT SHOULD be identical in the lab and in production +networks. + +# Acknowledgements + +Many thanks to Alec Hothan of OPNFV NFVbench project for thorough +review and numerous useful comments and suggestions. + +--- back diff --git a/docs/ietf/process.txt b/docs/ietf/process.txt index e170352cb9..261756fc8a 100644 --- a/docs/ietf/process.txt +++ b/docs/ietf/process.txt @@ -19,4 +19,6 @@ $ kdrfc --version $ sudo gem install kramdown-rfc2629 Main: -$ kdrfc draft-ietf-bmwg-mlrsearch-02.md +$ kdrfc draft-ietf-bmwg-mlrsearch-03.md + +If that complains, do it manually at https://author-tools.ietf.org/
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