linux-kernel - Re: [PATCH] Give kjournald a IOPRIO_CLASS

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Date:	Thu, 02 Oct 2008 17:32:04 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

Jens Axboe <jens.axboe@...cle.com> wrote:
> On Thu, Oct 02 2008, Dave Chinner wrote:
>> On Thu, Oct 02, 2008 at 09:55:11AM +0200, Jens Axboe wrote:

>> > Good point. I think we should mark the IO as sync, and maintain the same
>> > priority level. Any IO that ends up being waited on is sync by
>> > definition, we just need to expand the coverage a bit.
>> 
>> That's what XFS has always done - mark the journal I/O as sync.
>> Still, once you load up the elevator, the sync I/O can still get
>> delayed for hundreds of milliseconds before dispatch, which was
>> why I started looking at boosting the priority of the log I/O.
>> It proved to be much more effective at getting the log I/O
>> dispatched than the existing "mark it sync" technique....
> 
> Sure, just marking it as sync is not a magic bullet. It'll be in the
> first priority for that class, but it'll share bandwidth with other
> processes. So if you have lots of IO going on, it can take hundreds of
> miliseconds before being dispatched.

Sounds like you need a priority class besides sync and async.


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