linux-kernel - Re: [PATCH] perf record: Allow poll timeout to be specified

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Message-ID: <55118E5A.20803@oracle.com>
Date:	Tue, 24 Mar 2015 10:18:34 -0600
From:	David Ahern <david.ahern@...cle.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	acme@...nel.org, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH] perf record: Allow poll timeout to be specified

On 3/24/15 10:12 AM, Ingo Molnar wrote:
>
> * David Ahern <david.ahern@...cle.com> wrote:
>
>> Record currently wakes up based on watermarks to read events from
>> the mmaps and write them out to the file. The result is a file that
>> can have large blocks of events per mmap before a finished round
>> event is added to the stream.  This in turn affects the quantity of
>> events that have to be passed through the ordered events queue
>> before results can be displayed to the user. For commands like
>> perf-script this can lead to long unnecessarily long delays before a
>> user gets output. Large systems (e.g, 1024 cpus) further compound
>> this effect. I have seen instances where I have to wait 45 minutes
>> for perf-script to process a 5GB file before any events are shown.
>>
>> This patch adds an option to perf-record to allow a user to specify
>> the poll timeout in msec. For example using 100 msec timeouts
>> similar to perf-top means the mmaps are traversed much more
>> frequently leading to a smoother analysis side.
>
> Please tune the default value (perhaps influenced by N_PROC?) so that
> users will get sane behavior without having to specify this option!

I knew you were going to say that! ;-)

It's really a function of events coming in not cpus. The number of CPUs 
just compounds the problem.

I thought about making perf-record use a 100msec timeout like perf-top, 
but that can lead to unnecessary FINISHED_ROUND events in the file and 
unnecessary noise/overhead in the record side. On the other hand looking 
at scheduler tracepoints, kvm tracepoints, etc -- those can flood in to 
the point that even 100msec is too long.
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