linux-kernel - Re: Scheduler accounting inflated for io bound processes.

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Message-ID: <51CB110E.6010707@gmail.com>
Date:	Wed, 26 Jun 2013 10:04:30 -0600
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <bitbucket@...ine.de>,
	Dave Chiluk <chiluk@...onical.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Scheduler accounting inflated for io bound processes.

On 6/26/13 9:50 AM, Ingo Molnar wrote:
>
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Wed, Jun 26, 2013 at 11:37:13AM +0200, Ingo Molnar wrote:
>>> Would be very nice to randomize the sampling rate, by randomizing the
>>> intervals within a 1% range or so - perf tooling will probably recognize
>>> the different weights.
>>
>> You're suggesting adding noise to the regular kernel tick?
>
> No, to the perf interval (which I assumed Mike was using to profile this?)
> - although slightly randomizing the kernel tick might make sense as well,
> especially if it's hrtimer driven and reprogrammed anyway.
>
> I might have gotten it all wrong though ...

Sampled S/W events like cpu-clock have a fixed rate 
(perf_swevent_init_hrtimer converts freq to sample_period).

Sampled H/W events have an adaptive period that converges to the desired 
sampling rate. The first few samples come in 10 usecs are so apart and 
the time period expands to the desired rate. As I recall that adaptive 
algorithm starts over every time the event is scheduled in.

David
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