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    SubjectRe: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels

    * stephane eranian <> wrote:

    > 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.

    Btw., did we strongly oppose the notion? The mmap output
    buffers were always multi-event in essence - just by
    virtue of the automatic events such as task creation,
    mmap, etc.

    > 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 you want to recover the momentary index for RDPC you
    need to mmap the counter(s) you are interested in, to get
    to the header->index value.

    So you can create these counters with 1 pages only (one
    is the header) - and then remap the output to the primary
    counter's output buffer.

    > - 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.

    Via PERF_SAMPLE_ID. That saves the counter ID into the
    event so it can be demultiplexed later on.


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