linux-kernel - Re: [PATCH 2/2] cfq: allow dispatching of both sync and async I/O together

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Message-ID: <x49eig06tip.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 21 Jun 2010 16:05:34 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cfq: allow dispatching of both sync and async I/O together

Jens Axboe <axboe@...nel.dk> writes:

> On 21/06/10 21.49, Jeff Moyer wrote:
>> Hi,
>> 
>> In testing a workload that has a single fsync-ing process and another
>> process that does a sequential buffered read, I was unable to tune CFQ
>> to reach the throughput of deadline.  This patch, along with the previous
>> one, brought CFQ in line with deadline when setting slice_idle to 0.
>> 
>> I'm not sure what the original reason for not allowing sync and async
>> I/O to be dispatched together was.  If there is a workload I should be
>> testing that shows the inherent problems of this, please point me at it
>> and I will resume testing.  Until and unless that workload is identified,
>> please consider applying this patch.
>
> The problematic case is/was a normal SATA drive with a buffered
> writer and an occasional reader. I'll have to double check my
> mail tomorrow, but iirc the issue was that the occasional reader
> would suffer great latencies since service times for that single
> IO would be delayed at the drive side. It could perhaps just be
> a bug in how we handle the slice idling on the read side when the
> IO gets delayed initially.
>
> So if my memory is correct, google for the fsync madness and
> interactiveness thread that we had some months ago and which
> caused a lot of tweaking. The commit adding this is
> 5ad531db6e0f3c3c985666e83d3c1c4d53acccf9 and was added back
> in July last year. So it was around that time that the mails went
> around.

OK.  Thanks a ton for the pointers!  I really appreciate it!

-Jeff
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