1 .. _latency_methodology:
6 TRex Traffic Generator (TG) is used for measuring one-way latency in
7 2-Node and 3-Node physical testbed topologies. TRex integrates `High
8 Dynamic Range Histogram (HDRH) <http://hdrhistogram.org/>`_
9 functionality and reports per packet latency distribution for latency
10 streams sent in parallel to the main load packet streams.
12 Following methodology is used:
14 - Only NDRPDR test type measures latency and only after NDR and PDR
15 values are determined. Other test types do not involve latency
17 - Latency is measured at different background load packet rates:
19 - No-Load: latency streams only.
20 - Low-Load: at 10% PDR.
21 - Mid-Load: at 50% PDR.
22 - High-Load: at 90% PDR.
24 - Latency is measured for all tested packet sizes except IMIX due to
26 - TG sends dedicated latency streams, one per direction, each at the
27 rate of 9 kpps at the prescribed packet size; these are sent in
28 addition to the main load streams.
29 - TG reports Min/Avg/Max and HDRH latency values distribution per stream
30 direction, hence two sets of latency values are reported per test case
31 (marked as E-W and W-E).
32 - +/- 1 usec is the measurement accuracy of TRex TG and the data in HDRH
33 latency values distribution is rounded to microseconds.
34 - TRex TG introduces a (background) always-on Tx + Rx latency bias of 4
35 usec on average per direction resulting from TRex software writing and
36 reading packet timestamps on CPU cores. Quoted values are based on TG
37 back-to-back latency measurements.
38 - Latency graphs are not smoothed, each latency value has its own
39 horizontal line across corresponding packet percentiles.
40 - Percentiles are shown on X-axis using a logarithmic scale, so the
41 maximal latency value (ending at 100% percentile) would be in
42 infinity. The graphs are cut at 99.9999% (hover information still