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    SubjectRe: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels
    On Wed, Aug 19, 2009 at 2:56 PM, Peter Zijlstra<> 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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