linux-kernel - Re: [PATCHSET 00/16] perf top: Add multi-thread support (v1)

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Date:	Mon, 14 Dec 2015 10:26:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Namhyung Kim <namhyung@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCHSET 00/16] perf top: Add multi-thread support (v1)

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@...il.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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