.. _latency_methodology: Packet Latency -------------- 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) `_ functionality and reports per packet latency distribution for latency streams sent in parallel to the main load packet streams. 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 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%).