Date: Mon, 14 Dec 2015 10:26:13 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>, Namhyung Kim <namhyung@gmail.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Andi Kleen <andi@firstfloor.org>,
        Stephane Eranian <eranian@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>
Subject: Re: [PATCHSET 00/16] perf top: Add multi-thread support (v1)
Message-ID: <20151214092613.GL6357@twins.programming.kicks-ass.net>
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On Fri, Dec 11, 2015 at 08:01:31AM -0700, David Ahern wrote:
> On 12/11/15 1:11 AM, Ingo Molnar wrote:
> >
> >* Namhyung Kim <namhyung@gmail.com> wrote:
> >
> >>IIRC David said that thread per cpu seems too much especially on a large system
> >>(like ~1024 cpu). [...]
> >
> >Too much in what fashion? For recording I think it's the fastest, most natural
> >model - anything else will create cache line bounces.
> 
> The intrusiveness of perf on the system under observation. I understand
> there are a lot of factors that go into it.

So I can see some of that, if every cpu has its own thread then every
cpu will occasionally schedule that thread. Whereas if there were less,
you'd not have that.

Still, I think it makes sense to implement it, we need the multi-file
option anyway. Once we have that, we can also implement a per-node
option, which should be a fairly simple hybrid of the two approaches.

The thing is, perf-record is really struggling on big machines.

And in an unrelated note, I absolutely detest --buildid being the
default, it makes perf-record blow chunks.
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