linux-kernel - Re: [PATCH] cfq-iosched: don't allow aliased requests to starve others

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Message-ID: <x49iq42gz65.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 26 Jul 2010 09:17:38 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Heinz Diehl <htd@...cy-poultry.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cfq-iosched: don't allow aliased requests to starve  others

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

> On 07/24/2010 10:04 AM, Heinz Diehl wrote:
>> On 14.07.2010, Jeff Moyer wrote: 
>> 
>>> Comments, as always, are welcome.
>> 
>> This patch, applied to 2.6.35-rc6, increases desktop interactivity
>> _NOTICEABLY_ on my quadcore machine, and the machine stays rock-stable.
>> I have now tested this patch with the latest 2.6.35-rc kernels over 
>> 1 week.
>> 
>> Unfortunately, I can't provide some testing results which makes this
>> statement more objective, but I'll do some synthetic testing in the next
>> days.
>
> It is extremely unlikely that this patch will have any impact on
> "normal" workloads. To even hit a code path where it would make a
> difference, you would need to use O_DIRECT IO, otherwise you cannot have
> aliases in the IO scheduler.

I agree that it shouldn't help normal workloads at all.  I do think
there is one other case where you can get aliases: doing I/O both
through the file system and the underlying device.  However, that's
obviously a bad idea (and maybe open_bdev_exclusive will keep that from
happening?).

Cheers,
Jeff
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