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+---
+bookToc: false
+title: "Packet Latency"
+weight: 8
+---
+
+# 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)](http://hdrhistogram.org/)
+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%).
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