Messages in this thread | | | Date | Thu, 20 Oct 2005 16:11:04 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH linux-2.6-block:master 02/05] blk: update ioscheds to use generic dispatch queue |
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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
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