linux-kernel - Re: [PATCH 1/2] perf tools: Add option to copy events when queueing

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Date:	Fri, 03 Oct 2014 18:25:15 +0400
From:	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] perf tools: Add option to copy events when queueing

Ingo Molnar <mingo@...nel.org> writes:

> * Jiri Olsa <jolsa@...hat.com> wrote:
>
>> On Fri, Oct 03, 2014 at 06:34:21AM +0200, Ingo Molnar wrote:
>> > 
>> > * Alexander Yarygin <yarygin@...ux.vnet.ibm.com> wrote:
>> > 
[..]
>> > 
>> > What's the performance effect of this - i.e. by how much does CPU 
>> > use increase due to copying the events?
>> > 
>> > Wouldn't it be faster to fix this problem by updating the mmap 
>> > tail pointer only once the event has truly been consumed?
>> 
>> Alexander mentioned he'd loose data, because of userspace 
>> processing being to slow:
>>
>> http://marc.info/?l=linux-kernel&m=141111652424818&w=2
>
> So copying helps by allocating an essentially larger buffer, to 
> hold all unprocessed events that user-space is too slow to 
> process?
>
> I guess it's a valid usecase.
>
> Thanks,
>
> 	Ingo

Right. Also, it looks like the overhead here isn't a big deal:
time needed for actual processing an event is significantly bigger
and the additional memdup() doesn't change that much.

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