Messages in this thread | | | From | Andi Kleen <> | Subject | Optimize perf stat for large number of events/cpus | Date | Thu, 7 Nov 2019 10:16:33 -0800 |
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[v5: Address review feedback. Split patches.]
This patch kit optimizes perf stat for a large number of events on systems with many CPUs and PMUs.
Some profiling shows that the most overhead is doing IPIs to all the target CPUs. We can optimize this by using sched_setaffinity to set the affinity to a target CPU once and then doing the perf operation for all events on that CPU. This requires some restructuring, but cuts the set up time quite a bit.
In theory we could go further by parallelizing these setups too, but that would be much more complicated and for now just batching it per CPU seems to be sufficient. At some point with many more cores parallelization or a better bulk perf setup API might be needed though.
In addition perf does a lot of redundant /sys accesses with many PMUs, which can be also expensve. This is also optimized.
On a large test case (>700 events with many weak groups) on a 94 CPU system I go from
real 0m8.607s user 0m0.550s sys 0m8.041s
to
real 0m3.269s user 0m0.760s sys 0m1.694s
so shaving ~6 seconds of system time, at slightly more cost in perf stat itself. On a 4 socket system with the savings are more dramatic:
real 0m15.641s user 0m0.873s sys 0m14.729s
to
real 0m4.493s user 0m1.578s sys 0m2.444s
so 11s difference in the user visible set up time.
Also available in
git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc perf/stat-scale-8
v1: Initial post. v2: Rebase. Fix some minor issues. v3: Rebase. Address review feedback. Fix one minor issue v4: Modified based on review feedback. Now it maintains all_cpus per evlist. There is still a need for cpu_index iteration to get the correct index for indexing the file descriptors. Fix bug with unsorted cpu maps, now they are always sorted. Some cleanups and refactoring. v5: Split patches. Redo loop iteration again. Fix cpu map merging for uncore. Remove duplicates from cpumaps. Add unit tests.
-Andi
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