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

[Jens Axboe <axboe@suse.de>]

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?

-- 
Jens Axboe