Reduce timing overhead of EXPLAIN ANALYZE using rdtsc #16
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INSTR_TIME_SET_SECONDS() is useful to calculate the end of a time-bound loop without having to convert into time units (which is costly). INSTR_TIME_IS_LT() can be used to check the loop condition. Author: Reviewed-by: Discussion: https://postgr.es/m/ Backpatch:
This commit also updates pg_test_timing's documentation: - compare EXPLAIN (ANALYZE, TIMING ON/OFF) instead of comparing performance of of statement with/without EXPLAIN ANALYZE - explain the 2x overhead (due to two timestamp acquisitions per row) - remove old section about old versions of linux - I couldn't update the numbers, and it's old enough nobody would care
We switch to using the time stamp counter (TSC) instead of clock_gettime() to reduce overhead of EXPLAIN (ANALYZE, TIME ON). Tests showed that runtime is reduced by around 10% for queries moving lots of rows through the plan. For now this is only enabled on Linux/x86, in case the system clocksource is reported as TSC. Relying on the Linux kernel simplifies the logic to detect if the present TSC is usable (frequency invariant, synchronized between sockets, etc.). In all other cases we fallback to clock_gettime(). Note, that we intentionally use RDTSC in the fast paths, rather than RDTSCP. RDTSCP waits for outstanding instructions to retire on out-of-order CPUs. This adds noticably for little benefit in the typical InstrStartNode() / InstrStopNode() use case. The macro to be used in such cases is called INSTR_TIME_SET_CURRENT_FAST(). The original macro INSTR_TIME_SET_CURRENT() uses RDTSCP and is supposed to be used when precision is more important than performance. Author: David Geier <[email protected]> Author: Andres Freund <[email protected]> Author: Lukas Fittl <[email protected]> Reviewed-by: Discussion: https://www.postgresql.org/message-id/flat/20200612232810.f46nbqkdhbutzqdg%40alap3.anarazel.de
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RDTSC resources:
TODO:
kvm-clocksource)