linux-kernel - Re: [RFC/RFT PATCH] cfq-iosched: Implement cfq group idling

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Date:	Thu, 8 Jul 2010 17:15:17 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [RFC/RFT PATCH] cfq-iosched: Implement cfq group idling

On Thu, Jul 8, 2010 at 4:08 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
>
>> On Thu, Jul 08, 2010 at 09:39:45AM -0400, Jeff Moyer wrote:
>>> Vivek Goyal <vgoyal@...hat.com> writes:
>>>
>>> > Currently we idle on sequential queues and allow dispatch from a single
>>> > queue and that can become a bottleneck on higher end storage. For example
>>> > on my HP EVA, I can run multiple sequential streams and achieve top BW
>>> > of around 350 MB/s. But with CFQ, dispatching from single queue does not
>>> > keep the array busy (limits to 150-180 MB/s with 4 or 8 processes).
>>> >
>>> > One approach to solve this issue is simply use slice_idle = 0. But this
>>> > also takes away the any service differentiation between groups.
>>>
>>> That also takes away service differentiation between queues.  If you
>>> want to maintain that at all, then this is really just pushing the
>>> problem to another layer.
>>>
>>
>> Yes it does take away the io priority with-in group. But I think that's
>> the trade-off and that's not default. So those who don't require ioprio
>> stuff working with-in group and those who know that they have got
>> faster storage will set slice_idle=0. For rest of the SATA users default
>> is still slice_idle=8.
>
> [snip]
>
> Sorry, Vivek, I'm actually hijacking your thread.  ;-)  I know what the
> alternatives are, what I'm looking for is guidance on what Jens wants to
> do with CFQ.  We can discuss the merits of different approaches once we
> agree on a set of requirements.
>
> Cheers,
> Jeff
>

Hi Jeff,
did you have a look at:
http://lkml.indiana.edu/hypermail/linux/kernel/1004.3/00082.html (and
following msgs)?
We tried to achieve better throughput on multi-spindle disks, by merging queues.
I think the idea is promising, but we still need a lot of work to make
it concrete.
If you want to hack on it, feel free, since I don't have hardware to test it.

Corrado

__________________________________________________________________________

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