Messages in this thread | | | Date | Thu, 20 Aug 2009 12:13:25 +0200 | Subject | Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels | From | stephane eranian <> |
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On Wed, Aug 19, 2009 at 2:56 PM, Peter Zijlstra<a.p.zijlstra@chello.nl> wrote: > On Wed, 2009-08-19 at 22:36 +1000, Paul Mackerras wrote: >> Peter Zijlstra writes: >> >> > Provide the ability to configure a counter to send its output to >> > another (already existing) counter's output stream. >> >> What sort of thing might this be useful for? > > Some people complained that its tedious to mmap() for every counter and > would like to share the mmap() output buffer between counters. >
> This saves on address space and mlock budget and I guess fd management > logic. > Interesting to see, you seem to have changed your mind on this. I recall pointing this out in my early comments.
But anyway, here are some more comments:
- how does this work with the remapped counts? Probably only see the count for the target, i.e., output, event
- if samples from multiple events end up in the same buffer, how do I tell them apart, i.e., how do I know sample X came from event A, sample X from event B? This may be useful to detect patterns.
> As long as you're not mixing counters for different tasks/cpus there > should be no performance penalty, but even if you do that it might work > well enough on slow samples/small systems.. > >> Does this only apply to sampling counters? > > Yeah, everything that would otherwise go through the mmap() buffer. > > I'm not sure there's anything to be done about the read(2) thing. > >
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