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

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Date:	Tue, 22 Jun 2010 08:45:54 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cfq: allow dispatching of both sync and async I/O together

Vivek Goyal <vgoyal@...hat.com> writes:

> On Mon, Jun 21, 2010 at 07:22:08PM -0400, Vivek Goyal wrote:
>> On Mon, Jun 21, 2010 at 09:59:48PM +0200, Jens Axboe wrote:
>> > 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.
>> 
>> Hi Jens,
>> 
>> I suspect we might have introduced this patch because mike galbraith
>> had issues which application interactiveness (reading data back from swap)
>> in the prence of heavy writeout on SATA disk.
>> 
>> After this patch we did two enhancements.
>> 
>> - You introduced the logic of building write queue depth gradually.
>> - Corrado introduced the logic of idling on the random reader service
>>   tree.
>> 
>> In the past random reader were not protected from WRITES as there was no
>> idling on random readers. But with corrado's changes of idling on
>> sync-noidle service tree, I think this problem might have been solved to
>> a great extent.
>> 
>> Getting rid of this exclusivity of either SYNC/ASYNC requests in request
>> queue might help us with throughput on storage arrys without loosing
>> protection for random reader on SATA. 
>> 
>> I will do some testing with and without patch and see if above is true
>> or not.
>
> Some primilinary testing results with and without patch. I started a
> buffered writer and started firefox and monitored how much time firefox
> took.
>
> dd if=/dev/zero of=zerofile bs=4K count=1024M
>
> 2.6.35-rc3 vanilla
> ==================
> real    0m22.546s
> user    0m0.566s
> sys     0m0.107s
>
>
> real    0m21.410s
> user    0m0.527s
> sys     0m0.095s
>
>
> real    0m27.594s
> user    0m1.256s
> sys     0m0.483s
>
> 2.6.35-rc3 + jeff's patches
> ===========================
> real    0m20.372s
> user    0m0.635s
> sys     0m0.128s
>
> real    0m22.281s
> user    0m0.509s
> sys     0m0.093s
>
> real    0m23.211s
> user    0m0.674s
> sys     0m0.140s
>
> So looks like firefox launching times have not changed much in the presence
> of heavy buffered writting going on root disk. I will do more testing tomorrow.

Was the buffered writer actually hitting disk?  How much memory is on
your system?

Cheers,
Jeff
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