linux-kernel - Re: [RFD PATCH 10/10] sched: io

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Message-ID: <20141030144634.GH23531@worktop.programming.kicks-ass.net>
Date:	Thu, 30 Oct 2014 15:46:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, rafael.j.wysocki@...el.com, mingo@...nel.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
	mturquette@...aro.org, tuukka.tikkanen@...aro.org,
	nicolas.pitre@...aro.org, patches@...aro.org
Subject: Re: [RFD PATCH 10/10] sched: io_latency: Tracking via buckets

On Wed, Oct 22, 2014 at 03:57:53PM +0200, Daniel Lezcano wrote:
> The size of the bucket is the bucket interval and represent the resolution
> of the statistic model. Eg with a bucket interval of 1us, it leads us to
> do statitics on all numbers, with of course a bad prediction because the
> number of latencies is big. A big interval can give better statistics,
> but can give us a misprediction as the interval is larger.
> 
> Choosing the size of the bucket interval vs the idle sleep time is the
> tradeoff to find. With a 200us bucket interval, the measurements show
> we still have good predictions, less mispredictions and cover the idle
> state target residency.
> 

For the record:

The suggestion in DUS was to align the bucket sizes with the various
break even times of the actual C states.

I forgot who suggested this; it might have been Morten; but it seems to
make sense.
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