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

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Message-ID: <CAM9d7chehcrE2ZsvNThK-GeNTY_OChsPDkmm5NpJG5bwnKciaQ@mail.gmail.com>
Date:	Tue, 19 Nov 2013 01:48:43 +0000
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" <linux-kernel@...r.kernel.org>,
	Jiri Olsa <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 David,

On Tue, Nov 19, 2013 at 12:34 AM, David Ahern <dsahern@...il.com> 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.

Ah, I missed that part - just blindly answered about the freq -
thinking he's talking about the default freq of perf record/top.

Anyway, for above case, I guess it won't affect much as stack usually
is in memory so no page fault will occur even recording with mmap
unless the system suffers from a high memory pressure, right?

But I agree that copying 8KB for each sample seems too large.


-- 
Thanks,
Namhyung
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