Date:	Fri, 27 Nov 2009 10:03:35 +0100
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, mel@....ul.ie, efault@....de
Subject: Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

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.

Thanks,
Corrado

On Fri, Nov 27, 2009 at 9:23 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
> On Thu, Nov 26 2009, Corrado Zoccolo wrote:
>> The introduction of ramp-up formula for async queue depths has
>> slowed down dirty page reclaim, by reducing async write performance.
>> This patch improves the formula by considering the remaining slice.
>>
>> The new formula will allow more dispatches at the beginning of the
>> slice, reducing them at the end.
>> This will ensure that we achieve good throughput, without the risk of
>> overrunning the allotted timeslice.
>>
>> The threshold is automatically increased when sync I/O is not
>> intermingled with async, in accordance with the previous incarnation of
>> the formula.
>
> The slow ramp up is pretty much essential to being able to have low
> latency for the sync reads, so I'm afraid this will break that. I would
> prefer doing it through memory reclaim detection, like the other patch
> you and Motohiro suggested.
>
> --
> Jens Axboe
>
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
                               Tales of Power - C. Castaneda
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