linux-kernel - Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes

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Date:	Thu, 14 Nov 2013 15:44:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Joseph Schuchart <joseph.schuchart@...dresden.de>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	thomas.ilsche@...dresden.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in
 Passes

On Thu, Nov 14, 2013 at 07:26:06AM -0700, David Ahern wrote:
> On 11/14/13, 3:05 AM, Ingo Molnar wrote:
> 
> >What am I missing?
> 
> I have spent quite a bit of time on this problem on this well. I think the
> flush time needs to be based on the start time of each round, not the
> minimum time observed across mmaps. I have tried the minimum time stamp
> route and it still fails often enough to be annoying.
> 
> See builtin-kvm.c, perf_kvm__mmap_read(). The problem is that it takes time
> to move from mmap to mmap and sample can come in behind you - an mmap that
> has already be scanned with a timestamp less than what is determined to be
> the lowest minimum for the samples actually read. 'perf kvm stat live' in a
> nested environment is a stress test for the problem.

In which case you need the sliding sort window to be n*buf_size, where n
is the number of buffers flushed into the one file. Or move to one file
per buffer and merge sort the lot, buffers should be monotonic.


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