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

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Message-ID: <566ED644.2010601@gmail.com>
Date:	Mon, 14 Dec 2015 07:46:28 -0700
From:	David Ahern <dsahern@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 12/14/15 2:26 AM, Peter Zijlstra wrote:
> 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.

I've gone from the 1024-cpu sparc systems earlier this year down to 
small PPC and Rangeley-based switches. For both ends of the scale (and 
in between) I constantly struggle with the options to manage memory, cpu 
and disk consumption.

There definitely needs to be options (e.g., multi-threaded on/off). For 
the threading options I get the appeal for 1-thread per cpu but other 
options make sense as well -- 1 thread per core, 1 per NUMA node. perf 
has the CPU topology so should not be too difficult.

If you have 1-thread per cpu that means you are pinning the threads to 
the cpu? That brings in additional permissions problems.
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