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

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Date:	Wed, 25 Mar 2015 08:37:21 -0600
From:	David Ahern <david.ahern@...cle.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	acme@...nel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.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 3/25/15 6:38 AM, Jiri Olsa wrote:
> On Tue, Mar 24, 2015 at 12:09:48PM -0400, David Ahern 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.
>
> so you have pipe to perf script, right?

$ perf record ....
$ perf script ...
<wait an eternity>
data

>
>>
>> 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.
>
> there's also the '--no-buffering' option that sets:
>
>                  attr->watermark = 0;
>                  attr->wakeup_events = 1;
>
> but that's just the other edge, which is not what you'd want

right, that is the other extreme. record would never go to sleep.

>
> I think it's good to have user side configurable as well
>
> Acked-by: Jiri Olsa <jolsa@...nel.org>
>
> thanks,
> jirka
>

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