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