linux-kernel - Re: [PATCH V8 00/33] loop: Issue O_DIRECT aio using bio

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Date:	Mon, 5 Jan 2015 11:24:45 -0800
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Ming Lei <ming.lei@...onical.com>, <sedat.dilek@...il.com>
CC:	Dave Kleikamp <dave.kleikamp@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zach Brown <zab@...bo.net>
Subject: Re: [PATCH V8 00/33] loop: Issue O_DIRECT aio using bio_vec

On 12/31/2014 04:52 PM, Ming Lei wrote:
> On Thu, Jan 1, 2015 at 6:35 AM, Sedat Dilek <sedat.dilek@...il.com> wrote:
>> On Wed, Dec 31, 2014 at 10:52 PM, Dave Kleikamp
>> <dave.kleikamp@...cle.com> wrote:
>>> On 12/31/2014 02:38 PM, Sedat Dilek wrote:
>>>> What has happened to that aio_loop patchset?
>>>> Is it in Linux-next?
>>>> ( /me started to play with "block: loop: convert to blk-mq (v3)", so I
>>>> recalled this other improvement. )
>>> It met with some harsh resistance, so I backed off on it. Then Al Viro
>>> got busy re-writing the iov_iter infrastructure and I put my patchset on
>>> the shelf to look at later. Then Ming Lei submitted more up-to-date
>>> patchset: https://lkml.org/lkml/2014/8/6/175
>>>
>>> It looks like Ming is currently only pushing the first half of that
>>> patchset. I don't know what his plans are for the last three patches:
>>>
>>> aio: add aio_kernel_() interface
>>> fd/direct-io: introduce should_dirty for kernel aio
>>> block: loop: support to submit I/O via kernel aio based
>>>
>> I tested with block-mq-v3 (for next-20141231) [1] and this looks promising [2].
>>
>> Maybe Ming can say what the plan is with the missing parts.
> I have compared kernel aio based loop-mq(the other 3 aio patches
> against loop-mq v2, [1]) with loop-mq v3, looks the data isn't
> better than loop-mq v3.
>
> kernel aio based approach requires direct I/O, at least direct write
> shouldn't be good as page cache write, IMO.
>
> So I think we need to investigate kernel aio based approach further
> wrt. loop improvement.

A great advantage of kernel aio for loop device is the avoidance of 
double caching: the data from page cache of inner filesystem (mounted on 
the loop device) won't be cached again in the page cache of the outer 
filesystem (one that keeps image file of the loop device).

So I don't think it's correct to compare the performance of aio based 
loop-mq with loop-mq v3. Aio based approach is OK as long as it doesn't 
introduce significant overhead as compared with submitting bio-s 
straightforwardly from loop device (or any other in-kernel user of 
kernel aio) to host block device.

Thanks,
Maxim
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