linux-kernel - Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging

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Date:	Wed, 30 Dec 2009 22:21:08 +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>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging

On Wed, Dec 30, 2009 at 10:11 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
> On Wed, Dec 30 2009, Corrado Zoccolo wrote:
>> On Wed, Dec 30, 2009 at 7:45 PM, Jens Axboe <jens.axboe@...cle.com> wrote:
>> > On Wed, Dec 30 2009, Corrado Zoccolo wrote:
>> >> Non rotational devices' performances are not affected by
>> >> distance of requests, so there is no point in having overhead
>> >> to merge queues of nearby requests.
>> >
>> > If the distance is zero, it may still make a big difference (at least
>> > for writes). This check would be better as "ncq and doesn't suck", ala
>> >
>> >        blk_queue_nonrot(q) && tagged
>> >
>> > like we do elsewhere.
>>
>> For reads, though, even flash cards and netbook ssds are completely
>> unaffected. I have done few experiments on my available disks:
>> * http://dl.dropbox.com/u/3525644/service_time.png (I used the
>> program: http://dl.dropbox.com/u/3525644/stride.c to get the graphs).
>
> Completely agree, it's writes that matter (as mentioned).
>
>> For distance 0, I think request merging will be more effective than
>> queue merging, moreover I think the multi-thread trick to have large
>
> Definitely true, but we don't allow cross cfqq merges to begin with.
>
>> I/O depth is used for reads, not writes (where simply issuing buffered
>> writes already achieves a similar effect), so I think it is safe to
>> disable it for all non-rotational devices.
>
> That still leaves direct writes. Granted it's a problem with a huge
> scope, but still.
Maybe I can mark sync queues that have write requests, and only add
those ones to the prio tree.
For writes, merging queues (and therefore requests) can probably help
even the smart ssds.

Thanks,
Corrado
>
> --
> Jens Axboe
>
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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