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From: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>, linux-kernel@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] perf tools: Add option to copy events when queueing
References: <1412267936-19827-1-git-send-email-yarygin@linux.vnet.ibm.com>
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	<20141003084754.GA29966@gmail.com>
Date: Fri, 03 Oct 2014 18:25:15 +0400
In-Reply-To: <20141003084754.GA29966@gmail.com> (Ingo Molnar's message of
	"Fri, 3 Oct 2014 10:47:54 +0200")
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Ingo Molnar <mingo@kernel.org> writes:

> * Jiri Olsa <jolsa@redhat.com> wrote:
>
>> On Fri, Oct 03, 2014 at 06:34:21AM +0200, Ingo Molnar wrote:
>> > 
>> > * Alexander Yarygin <yarygin@linux.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.