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

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Message-ID: <871u2dup1x.fsf@sejong.aot.lge.com>
Date:	Tue, 19 Nov 2013 11:02:18 +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 17:34:49 -0700, David Ahern wrote:
> 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.

How about adding an option to exclude the perf tools from recording for
system-wide (or cpu-wide) session?

This way, we can prevent the feedback loops for page-fault or syscall
events you mentioned IMHO.

Thanks,
Namhyung
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