lkml.org 
[lkml]   [2013]   [Nov]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 4/5] perf record: mmap output file - v5
On Tue, Nov 19, 2013 at 2:33 AM, David Ahern <dsahern@gmail.com> 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


\
 
 \ /
  Last update: 2013-11-19 04:01    [W:1.987 / U:0.532 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site