report: Update packet latency methodology

Change-Id: I0378e96c6f64a1dd035d56465710b82f64dc4d32
Signed-off-by: Maciek Konstantynowicz <mkonstan@cisco.com>

1 files changed, 26 insertions, 18 deletions

diff --git a/docs/report/introduction/methodology_packet_latency.rst b/docs/report/introduction/methodology_packet_latency.rst
index 1f7ad7f633..d5486786b6 100644
--- a/docs/report/introduction/methodology_packet_latency.rst
+++ b/docs/report/introduction/methodology_packet_latency.rst
@@ -1,33 +1,41 @@
 Packet Latency
 --------------
 
-TRex Traffic Generator (TG) is used for measuring latency across 2-Node
-and 3-Node SUT server topologies. TRex integrates `A High Dynamic Range
-Histogram (HDRH) <http://hdrhistogram.org/>`_ code providing per packet
-latency distribution for latency streams sent in parallel to the main
-load packet streams. Packet latency is measured using following
-methodology:
+TRex Traffic Generator (TG) is used for measuring one-way latency in
+2-Node and 3-Node physical testbed topologies. TRex integrates `High
+Dynamic Range Histogram (HDRH) <http://hdrhistogram.org/>`_
+functionality and reports per packet latency distribution for latency
+streams sent in parallel to the main load packet streams.
 
-- Latency tests are performed at following packet load levels:
+Following methodology is used:
+
+- Only NDRPDR test type measures latency and only after NDR and PDR
+  values are determined. Other test types do not involve latency
+  streams.
+- Latency is measured at different background load packet rates:
 
   - No-Load: latency streams only.
   - Low-Load: at 10% PDR.
   - Mid-Load: at 50% PDR.
   - High-Load: at 90% PDR.
-  - NDR-Load: at 100% NDR.
-  - PDR-Load: at 100% PDR.
 
 - Latency is measured for all tested packet sizes except IMIX due to
-  TG restriction.
+  TRex TG restriction.
 - TG sends dedicated latency streams, one per direction, each at the
   rate of 9 kpps at the prescribed packet size; these are sent in
   addition to the main load streams.
 - TG reports Min/Avg/Max and HDRH latency values distribution per stream
-  direction, hence two sets of latency values are reported per test
-  case.
-- Reported latency values are aggregate across tested topology.
-- +/- 1 usec is the measurement accuracy advertised by TRex TG for the
-  setup used.
-- TG setup introduces an always-on Tx/Rx interface latency of about 2
-  * 2 usec per direction induced by TRex SW writing and reading packet
-  timestamps on CPU cores.
+  direction, hence two sets of latency values are reported per test case
+  (marked as E-W and W-E).
+- +/- 1 usec is the measurement accuracy of TRex TG and the data in HDRH
+  latency values distribution is rounded to microseconds.
+- TRex TG introduces a (background) always-on Tx + Rx latency bias of 4
+  usec on average per direction resulting from TRex software writing and
+  reading packet timestamps on CPU cores. Quoted values are based on TG
+  back-to-back latency measurements.
+- Latency graphs are not smoothed, each latency value has its own
+  horizontal line across corresponding packet percentiles.
+- Percentiles are shown on X-axis using a logarithmic scale, so the
+  maximal latency value (ending at 100% percentile) would be in
+  infinity. The graphs are cut at 99.9999% (hover information still
+  lists 100%).
\ No newline at end of file