Messages in this thread |  | | From | Namhyung Kim <> | Date | Tue, 19 Nov 2013 01:48:43 +0000 | Subject | Re: [PATCH 4/5] perf record: mmap output file - v5 |
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Hi David,
On Tue, Nov 19, 2013 at 12:34 AM, David Ahern <dsahern@gmail.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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