linux-kernel - Re: [PATCH v3 1/n] perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

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Message-ID: <20170619150939.GA4555@leverpostej>
Date:   Mon, 19 Jun 2017 16:09:39 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/n] perf/core: addressing 4x slowdown during
 per-process profiling of STREAM benchmark on Intel Xeon Phi

On Mon, Jun 19, 2017 at 07:59:08AM -0700, Andi Kleen wrote:
> > > For comparison, can you give --per-thread a go prior to these patches
> > > being applied?
> > 
> > FWIW, I had a go with (an old) perf record on an arm64 system using
> > --per-thread, and I see that no samples are recorded, which seems like a
> > bug.
> > 
> > With --per-thread, the slwodown was ~20%, whereas with the defaults it
> > was > 400%.
> 
> I'm not sure what the point of the experiment is? It has to work
> with reasonable overead even without --per-thread.
> 
> FWIW Alexey already root caused the problem, so there's no need
> to restart the debugging.

Sure; we understand where that overhead is coming from, we have an idea
as to how to mitigate that, and we should try to make that work it we
can.

I was trying to get a feel for how that compares to what we can do
today. For other reasons (e.g. fd exhaustion), opening NR_CPUS * n
events might not be a great idea on systems with a huge number of CPUs.
We might want a heuristic in the perf tool regardless.

Thanks,
Mark.