Messages in this thread |  | | From | Namhyung Kim <> | Date | Tue, 19 Nov 2013 02:36:42 +0000 | Subject | Re: [PATCH 4/5] perf record: mmap output file - v5 |
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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
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