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

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Date:	Tue, 19 Nov 2013 09:24:48 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Ahern <dsahern@...il.com>,
	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

Hi Ingo,

On Mon, 18 Nov 2013 10:40:36 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Fri, Nov 15, 2013 at 09:41:53AM -0700, David Ahern wrote:
>> > On 11/13/13, 4:34 AM, Peter Zijlstra wrote:
>> > >>>one option here is not allow page faults and system wide system calls.
>> > >>>system wide tracing needs mmap; page faults for a task can use write().
>> > >>>I left that option in case something like this came up.
>> > >>
>> > >>So maybe splice() sounds like the right long term solution after all? :-/
>> > >
>> > >Right until you put a tracepoint (kprobe) somewhere in whatever function
>> > >is used to transfer a single page into/from a splice pipe.
>> > >
>> > >You can always screw yourself over using this stuff, no exceptions.
>> > >
>> > 
>> > 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. :)

Thanks,
Namhyung
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