linux-kernel - Re: [PATCH] perf: Update event buffer tail when overwriting old events

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Message-ID: <20130709143150.GL25631@dyad.programming.kicks-ass.net>
Date:	Tue, 9 Jul 2013 16:31:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, eranian@...gle.com,
	ak@...ux.intel.com
Subject: Re: [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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