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

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Message-ID: <51CB1323.6090305@gmail.com>
Date:	Wed, 26 Jun 2013 10:13:23 -0600
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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 10:10 AM, Ingo Molnar wrote:
>> 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.
>
> Yes, but last I checked it (2 years ago? :-) the auto-freq code was
> converging pretty well to the time clock, with little jitter - in essence
> turning it into a fixed-period, fixed-frequency sampling method. That
> would explain Mike's results.

It does converge quickly and stay there for CPU-based events. My point 
was more along the lines that the code is there. Perhaps a tweak to add 
jitter to the period would address fixed period sampling affects.

David
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