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==== Latency ==== Pipeline latency can be measured at the link layer with an echo request/response (βpingβ) workload. An upper bound of pipeline latency can be established by measuring RTT between two devices that are directly connected (i.e., without an intermediate switch). In a two-machine test, full RTT is recorded to avoid requiring clocks synchronized at sub microsecond precision. A single machine setup with two devices (or ports on a single device) can report half-RTT. The reported value is an upper bound. 50/90/99 percentiles should be reported. Low event rate tests can be sensitive to cache and power scaling effects, as discussed in the section on reproducible results. This test alone cannot differentiate Rx from Tx latency. Hardware timestamps can be used to compute the time spent in the cable (and switch or wider fabric), to reduce the overestimation in the L2 ping test, and to disambiguate Rx from Tx latency. <span id="appendix-c-revision-history"></span>
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