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

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Date:	Mon, 18 Nov 2013 17:34:49 -0700
From:	David Ahern <dsahern@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>
CC:	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 11/18/13, 5:24 PM, Namhyung Kim wrote:
>>>> What now? Can we add the mmap path as an option?
>>>
>>> I'd say an option is always a possibility, but someone please try
>>> what happens if you use stupid large events (dwarf stack copies) on
>>> PERF_COUNT_SW_PAGE_FAULTS (.period=1) while recording with mmap().
>>>
>>> The other option is to simply disallow PERF_SAMPLE_STACK_USER for
>>> that event.
>>>
>>> Personally I think 8k copies for every event are way stupid anyway,
>>> that's a metric ton of data at a huge cost.
>>
>> Well, with 1 khz sampling of a single threaded workload it's 8MB per
>> second - that's 80 MB for 10 seconds profiling - not the end of the
>> world.
>
> We now use 4 khz sampling frequency by default, just FYI. :)

I think Peter is asking about:
     perf record -e faults -c 1 --call-graph dwarf,8192 -a -- sleep 1

And as expected it is a massive feedback spiraling out of control.

David
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