linux-kernel - Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

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Date:	Fri, 27 Nov 2009 16:05:37 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, efault@....de
Subject: Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

On Fri, Nov 27, 2009 at 12:48:47PM +0100, Jens Axboe wrote:
> On Fri, Nov 27 2009, Corrado Zoccolo wrote:
> > Hi Jens,
> > let me explain why my improved formula should work better.
> > 
> > The original problem was that, even if an async queue had a slice of 40ms,
> > it could take much more to complete since it could have up to 31
> > requests dispatched at the moment of expiry.
> > In total, it could take up to 40 + 16 * 8 = 168 ms (worst case) to
> > complete all dispatched requests, if they were seeky (I'm taking 8ms
> > average service time of a seeky request).
> > 
> > With your patch, within the first 200ms from last sync, the max depth
> > will be 1, so a slice will take at most 48ms.
> > My patch still ensures that a slice will take at most 48ms within the
> > first 200ms from last sync, but lifts the restriction that depth will
> > be 1 at all time.
> > In fact, after the first 100ms, a new async slice will start allowing
> > 5 requests (async_slice/slice_idle). Then, whenever a request
> > completes, we compute remaining_slice / slice_idle, and compare this
> > with the number of dispatched requests. If it is greater, it means we
> > were lucky, and the requests were sequential, so we can allow more
> > requests to be dispatched. The number of requests dispatched will
> > decrease when reaching the end of the slice, and at the end we will
> > allow only depth 1.
> > For next 100ms, you will allow just depth 2, and my patch will allow
> > depth 2 at the end of the slice (but larger at the beginning), and so
> > on.
> > 
> > I think the numbers by Mel show that this idea can give better and
> > more stable timings, and they were just with a single NCQ rotational
> > disk. I wonder how much improvement we can get on a raid, where
> > keeping the depth at 1 hits performance really hard.
> > Probably, waiting until memory reclaiming is noticeably active (since
> > in CFQ we will be sampling) may be too late.
> 
> I'm not saying it's a no-go, just that it invalidates the low latency
> testing done through the 2.6.32 cycle and we should re-run those tests
> before committing and submitting anything.
> 

Any chance there is a description of the tests that were used to
evaulate low_latency around?

> If the 'check for reclaim' hack isn't good enough, then that's probably
> what we have to do.
> 

It isn't good enough. I'll try variations of the same idea but the
initial tests were not promising at all.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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