Packet Latency
--------------
-TRex Traffic Generator (TG) is used for measuring latency of VPP DUTs.
-Reported latency values are 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 100% of discovered NDR and PDR rates
- for each throughput test and packet size (except IMIX).
+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.
+
+- Latency is measured for all tested packet sizes except IMIX due to
+ 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 latency values per stream direction, hence two
- sets of latency values are reported per test case; future release of
- TRex is expected to report latency percentiles.
-- Reported latency values are aggregate across two SUTs if the three
- node topology is used for given performance test; for per SUT latency,
- reported value should be divided by two.
-- 1usec is the measurement accuracy advertised by TRex TG for the setup
- used in FD.io labs used by CSIT project.
-- TRex setup introduces an always-on error of about 2*2usec per latency
- flow additonal Tx/Rx interface latency induced by TRex SW writing and
- reading packet timestamps on CPU cores without HW acceleration on NICs
- closer to the interface line.
+- TG reports Min/Avg/Max and HDRH latency values distribution per stream
+ 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%).
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