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

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Date:	Tue, 24 Mar 2015 17:12:11 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <david.ahern@...cle.com>
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


* 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!

Thanks,

	Ingo
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