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    SubjectRe: [PATCH] perf: Update event buffer tail when overwriting old events
    On Tue, Jul 09, 2013 at 09:52:17PM +0800, Yan, Zheng wrote:
    > On 07/09/2013 04:05 PM, Peter Zijlstra wrote:
    > > On Tue, Jul 09, 2013 at 03:05:41PM +0800, Yan, Zheng wrote:
    > >
    > >> Thank you for your help. I ran the same test, the results for regular case
    > >> are much better. But it still has about 1% overhead, probably because we
    > >> enlarge the ring_buffer structure, make it less cache friendly.
    > >>
    > >> origin with the patch
    > >> AVG 1000 1013
    > >> STDEV 13.4 15.0
    > >
    > > And this is the !overwrite case, right? I don't suppose you cured the logic
    > > Namhyung Kim pointed out? That should affect the overwrite case I suppose since
    > > it won't switch to perf_event_output_overwrite().
    >
    > yes, it's the overwrite case.

    So the most common case is the !overwrite one; we should ensure no significant
    regression there. The overwrite case isn't used that much because as you've
    found its really hard to use without a valid tail pointer. So I'm not too
    bothered about making the overwrite case a _little_ more expensive if that
    makes it far more usable.


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