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: <20170619155204.GA5626@leverpostej>
Date:   Mon, 19 Jun 2017 16:52:05 +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 08:39:18AM -0700, Andi Kleen wrote:
> On Mon, Jun 19, 2017 at 04:24:01PM +0100, Mark Rutland wrote:
> > On Mon, Jun 19, 2017 at 08:21:51AM -0700, Andi Kleen wrote:
> > > > 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
> > > 
> > > You just have to increase the fd limit. The 1024 fd default is just
> > > archaic for larger systems and doesn't really make any sense because
> > > it only controls very small amounts of kernel memory.
> > > 
> > > > 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.
> > > 
> > > But there's no alternative: we have to measure all CPUs with all events.
> > 
> > You can measure the process on all CPUs by using 1 event without a CPU
> > filter, rather than NR_CPUS events.
> 
> That wouldn't measure all threads, at least not with current perf core.

Ah; I missed the constraint imposed by perf_mmap().

For some reason I thought that was enforced by userspace only.

Sorry for the noise.

Mark.