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

[Jens Axboe <jaxboe@fusionio.com>]

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.

-- 
Jens Axboe