linux-kernel - Re: [PATCH 4/5] perf record: mmap output file

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Message-ID: <87ob5ht962.fsf@sejong.aot.lge.com>
Date:	Tue, 19 Nov 2013 11:30:45 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, jolsa@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5

On Mon, 18 Nov 2013 19:17:37 -0700, David Ahern wrote:
> On 11/18/13, 7:13 PM, Namhyung Kim wrote:
>> I think it should be
>>
>>    perf record -e cycles -F 4000 -e faults -c 1 --call-graph dwarf,8192 -a -- sleep 1
>>
>> (at least to generate the feedback spiral more efficiently..)
>
> you don't need the cycles. faults by itself works. Each event contains
> 2 pages of data in the sample. With mmap-based output a single
> sample (1 page fault in any process) generates 2-3 page faults by perf
> which cause 2-3 >8k samples to be generated, which generates faults,
> ....

But after perf touches all pages in ring-buffer and stack, it won't
generate page-faults for itself anymore, right?

Hmm.. thinking it again, perf has all ring-buffer pages in memory when
mmap() called, right?  If so why not doing something like MAP_POPULATE
so that it doesn't need to generate minor-faults?

Thanks,
Namhyung
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