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

[Namhyung Kim]

On Tue, Nov 19, 2013 at 2:33 AM, David Ahern <dsahern@xxxxxxxxx> wrote:
> On 11/18/13, 7:30 PM, Namhyung Kim wrote:
>>
>> On Mon, 18 Nov 2013 19:17:37 -0700, David Ahern wrote:
>>>
>>> On 11/18/13, 7:13 PM, Namhyung Kim wrote:
>>>>
>>>> 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..)
>>>
>>>
>>> you don't need the cycles. faults by itself works. Each event contains
>>> 2 pages of data in the sample. With mmap-based output a single
>>> sample (1 page fault in any process) generates 2-3 page faults by perf
>>> which cause 2-3 >8k samples to be generated, which generates faults,
>>> ....
>>
>>
>> But after perf touches all pages in ring-buffer and stack, it won't
>> generate page-faults for itself anymore, right?
>>
>> Hmm.. thinking it again, perf has all ring-buffer pages in memory when
>> mmap() called, right?  If so why not doing something like MAP_POPULATE
>> so that it doesn't need to generate minor-faults?
>
>
> This is mmap'ed output, not the ring buffers or its stack. As the output
> file grows, new pages are needed and those are allocated on access via page
> faults. The ftruncate only extends the file size, it does not allocate pages
> at that time.

Argh, I was completely confused! ;-)

Please feel free to ignore what I said here..

Thanks,
Namhyung
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