Re: [PATCH 4/5] perf record: mmap output file - v5

From: Namhyung Kim <namhyung@kernel.org>
To: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
	linux-kernel@vger.kernel.org, jolsa@redhat.com,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Mike Galbraith <efault@gmx.de>,
	Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5
Date: Tue, 19 Nov 2013 11:13:46 +0900	[thread overview]
Message-ID: <87wqk5t9yd.fsf@sejong.aot.lge.com> (raw)
In-Reply-To: <528AB229.6030603@gmail.com> (David Ahern's message of "Mon, 18 Nov 2013 17:34:49 -0700")

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

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..)

Well, I know that we don't support this now.  But wouldn't it make sense
to support this kind of thing?

Thanks,
Namhyung