linux-kernel - Re: [patch] iosched: prevent aliased requests from starving other I/O

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Message-ID: <4DE76F61.7060409@fusionio.com>
Date:	Thu, 2 Jun 2011 13:09:21 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch] iosched: prevent aliased requests from starving other
 I/O

On 2011-06-02 13:07, Jens Axboe wrote:
> On 2011-06-01 18:21, Jeff Moyer wrote:
>> Hi, Jens,
>>
>> If you recall, I posted an RFC patch for this back in July of last year:
>> http://lkml.org/lkml/2010/7/13/279
>>
>> The basic problem is that a process can issue a never-ending stream of
>> async direct I/Os to the same sector on a device, thus starving out
>> other I/O in the system (due to the way the alias handling works in both
>> cfq and deadline).  The solution I proposed back then was to start
>> dispatching from the fifo after a certain number of aliases had been
>> dispatched.  Vivek asked why we had to treat aliases differently at all,
>> and I never had a good answer.  So, I put together a simple patch which
>> allows aliases to be added to the rb tree (it adds them to the right,
>> though that doesn't matter as the order isn't guaranteed anyway).  I
>> think this is the preferred solution, as it doesn't break up time slices
>> in CFQ or batches in deadline.  I've tested it, and it does solve the
>> starvation issue.  Let me know what you think.
> 
> That'll work, there's no inherent reason why we can't have aliases
> directly in the rbtree as long as the sort insert factors that into
> account.
> 
> I will queue this one up for 3.1.

Jeff, care to resend a "proper" patch (signed-off and so forth)?

-- 
Jens Axboe

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