Date: Thu, 20 Oct 2005 16:41:08 +0200
From: Jens Axboe <axboe@suse.de>
To: Tejun Heo <htejun@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-2.6-block:master 02/05] blk: update ioscheds to use generic dispatch queue
Message-ID: <20051020144108.GR2811@suse.de>
References: <20051019123429.450E4424@htj.dyndns.org> <20051019123429.D377069C@htj.dyndns.org> <20051020112109.GC2811@suse.de> <20051020135124.GB26004@htj.dyndns.org> <20051020141104.GQ2811@suse.de> <4357AB3F.1050004@gmail.com>
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On Thu, Oct 20 2005, Tejun Heo wrote:
> 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?

That was the case before and I agree it's better to sort everything.
What I'm asking is when do you ever want to _not_ sort, unless you are
explicitly told to do INSERT_BACK? I don't mean the existing
list_add_tail() that got converted, those are clearly a win. And since
the _BACK handling is now generic, I don't see a need to pass in 'force'
for any other purpose than 'we really need to force requests out, don't
idle or anticipate, return what you have'.

Am I more clear now?

-- 
Jens Axboe

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