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

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Date:	Thu, 14 Nov 2013 08:02:48 -0700
From:	David Ahern <dsahern@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 11/14/13, 7:44 AM, Peter Zijlstra wrote:
> 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.

The one file per buffer does not work for live commands -- like perf-top 
and kvm-stat-live. perf-trace is not using the ordered samples code, but 
I think it needs to - especially for system wide.

David

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