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diff --git a/docs/content/methodology/test/reconfiguration.md b/docs/content/methodology/test/reconfiguration.md new file mode 100644 index 0000000000..a0678103d9 --- /dev/null +++ b/docs/content/methodology/test/reconfiguration.md @@ -0,0 +1,70 @@ +--- +title: "Reconfiguration" +weight: 8 +--- + +# Reconfiguration + +## Overview + +Reconf tests are designed to measure the impact of VPP re-configuration +on data plane traffic. +While VPP takes some measures against the traffic being +entirely stopped for a prolonged time, +the immediate forwarding rate varies during the re-configuration, +as some configurations steps need the active dataplane worker threads +to be stopped temporarily. + +As the usual methods of measuring throughput need multiple trial measurements +with somewhat long durations, and the re-configuration process can also be long, +finding an offered load which would result in zero loss +during the re-configuration process would be time-consuming. + +Instead, reconf tests first find a througput value (lower bound for NDR) +without re-configuration, and then maintain that ofered load +during re-configuration. The measured loss count is then assumed to be caused +by the re-configuration process. The result published by reconf tests +is the effective blocked time, that is +the loss count divided by the offered load. + +## Current Implementation + +Each reconf suite is based on a similar MLRsearch performance suite. + +MLRsearch parameters are changed to speed up the throughput discovery. +For example, PDR is not searched for, and the goal duration sum is shorter. +Contrary to usual NDR, exceed ratio is set to zero in reconf tests +as load stability is less important than attribution of losses. + +The MLRsearch suite has to contain a configuration parameter +that can be scaled up, e.g. number of tunnels or number of service chains. +Currently, only increasing the scale is supported +as the re-configuration operation. In future, scale decrease +or other operations can be implemented. + +The traffic profile is not changed, so the traffic present is processed +only by the smaller scale configuration. The added tunnels / chains +are not targetted by the traffic. + +For the re-configuration, the same Robot Framework and Python libraries +are used, as were used in the initial configuration, with the exception +of the final calls that do not interact with VPP (e.g. starting +virtual machines) being skipped to reduce the test overall duration. + +## Discussion + +Robot Framework introduces a certain overhead, which may affect timing +of individual VPP API calls, which in turn may affect +the number of packets lost. + +The exact calls executed may contain unnecessary info dumps, repeated commands, +or commands which change a value that do not need to be changed (e.g. MTU). +Thus, implementation details are affecting the results, even if their effect +on the corresponding MLRsearch suite is negligible. + +The lower bound for NDR is the only value safe to be used when zero packets lost +are expected without re-configuration. But different suites show different +"jitter" in that value. For some suites, the lower bound is not tight, +allowing full NIC buffers to drain quickly between worker pauses. +For other suites, lower bound for NDR still has quite a large probability +of non-zero packet loss even without re-configuration. |