[Devel] Re: [PATCH] cfq: async queue allocation per priority

[Jens Axboe]

On Thu, Jul 19 2007, Vasily Tarasov wrote:
> On Wed, 2007-07-18 at 20:51 +0200, Jens Axboe wrote:
> > On Wed, Jul 18 2007, Vasily Tarasov wrote:
> > > Jens, I think the last patch, that makes queues allocation per priority,
> > > has a problem.
> > > 
> > > If we have two processes with different ioprio_class, but the same
> > > ioprio_data, their async requests will fall into the same queue. I guess
> > > such behavior is not expected, because it's not right to put real-time
> > > requests and best-effort requests in the same queue.
> > > 
> > > The attached patch fixes the problem by introducing additional *cfqq
> > > fields on cfqd, pointing to per-(class,priority) async queues.
> > 
> > Ugh yes. I'm pretty tempted just to reinstate the cfqq hash again, it
> > used to be a clean up but now the it's not stacking up so well.
> > 
> 
> Hello, Jens,
> 
> From my humble point of view cfqq hash has two problems:
> 
> 1. It is excess data structure. All needed information can be obtained
> from other structures easily, so the presence of hash is a bit
> strange... I mean that it's aim is not obvious :)
> 
> 2. Hash hides from a developer a pretty important concept of CFQ: there
> are shared between processes per-priority async queues. I think the code
> is the best documentation, so the explicit async cfqq pointers at cfqd
> structure reveal this concept greatly.
> 
> Summary:
> 
> IMHO the hash revival is not very good way. However, this is of course
> fully in your competence to choose the right decision! ;)

Yeah, it's probably still better off without the hash. I'll play with it
a bit and see what comes of it.

-- 
Jens Axboe