linux-kernel - Re: Optimize perf stat for large number of events/cpus v2

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Date:   Tue, 22 Oct 2019 10:02:23 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     acme@...nel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org,
        eranian@...gle.com, kan.liang@...ux.intel.com, peterz@...radead.org
Subject: Re: Optimize perf stat for large number of events/cpus v2

On Sun, Oct 20, 2019 at 10:51:53AM -0700, Andi Kleen wrote:
> [The earlier v1 version had a lot of conflicts against some
> recent libperf changes in tip/perf/core. Resolve that and
> also fix some minor issues.]
> 
> 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-4
> 
> v1: Initial post.
> v2: Rebase. Fix some minor issues.

looks really helpful, I ack-ed 1st 2 patches,
I'll need more time for the rest

thanks,
jirka