Re: [PATCH linux-2.6-block:master 02/05] blk: update ioscheds touse generic dispatch queue

[Tejun Heo]

Jens Axboe wrote:
> On Thu, Oct 20 2005, Tejun Heo wrote:
>
> > On Thu, Oct 20, 2005 at 01:21:09PM +0200, Jens Axboe wrote:
> >
> > > On Wed, Oct 19 2005, Tejun Heo wrote:
> > >
> > > > 02_blk_generic-dispatch-queue-update-for-ioscheds.patch
> > > >
> > > > 	This patch updates all four ioscheds to use generic dispatch
> > > > 	queue.  There's one behavior change in as-iosched.
> > > >
> > > > 	* In as-iosched, when force dispatching
> > > > 	  (ELEVATOR_INSERT_BACK), batch_data_dir is reset to REQ_SYNC
> > > > 	  and changed_batch and new_batch are cleared to zero.  This
> > > > 	  prevernts AS from doing incorrect update_write_batch after
> > > > 	  the forced dispatched requests are finished.
> > > >
> > > > 	* In cfq-iosched, cfqd->rq_in_driver currently counts the
> > > > 	  number of activated (removed) requests to determine
> > > > 	  whether queue-kicking is needed and cfq_max_depth has been
> > > > 	  reached.  With generic dispatch queue, I think counting
> > > > 	  the number of dispatched requests would be more appropriate.
> > > >
> > > > 	* cfq_max_depth can be lowered to 1 again.
> > >
> > > I applied this one as well, with some minor changes. The biggest one is
> > > a cleanup of the 'force' logic, it seems to be a little mixed up in this
> > > patch. You use it for forcing dispatch, which is fine. But then it also
> > > doubles as whether you want to sort insert on the generic queue or just
> > > add to the tail?
> >
> > When forced dispatch occurs, all requests in a elevator get dumped
> > into the dispatch queue.  Specific elevators are free to dump in any
> > order and it's likely that specific elevators don't dump in the
> > optimal order - e.g. for cfq, it will dump each cfqq's in order which
> > results in unnecessary seeks.  That's why all the current ioscheds
> > tells elv_dispatch_insert() to perform global dispatch queue sorting
> > when they dump requests due to force argument.  Maybe add comments to
> > explain this?
>
>
> But why would you ever want non-sorted dispatch adding of requests,
> except for the cases where you absolutely need it to go at the back? I
> don't see what dispatch forcing has to do with this at all?

 For example, let's assume iosched is cfq.

 cfqq#0			cfqq#1

 4 5 8 9		3 6 7

 While operating normally, cfqq may dispatch 4, 5 for cfqq#0 and then 
(possibly after idle delay) 3, 6, 7 for cfqq#1.  In these cases, iosched 
is performing sort so it tells elv_dispatch_insert() to just append to 
the dispatch queue by setting @sort to zero.

 But, let's say a barrier request gets queued.  Core elevator code asks 
iosched to dump all requests it has.  For cfqq, it results in the 
following sequence.

 4 5 8 9 3 6 7 barrier

 Which isn't optimal.  As iosched's dispatching criteria also includes 
stuff like fairness / timing which can't be accounted for when forced 
dumping occurs, keeping the dumping order isn't very meaningful.  By 
setting @sort to 1 for forced dumps, we get,

 3 4 5 6 7 8 9 barrier

 Does this make sense to you?

--
tejun
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