Messages in this thread Patch in this message | | | Date | Tue, 27 May 2003 08:54:36 +0200 | From | Jens Axboe <> | Subject | Re: [BK PATCHES] add ata scsi driver |
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On Mon, May 26 2003, Linus Torvalds wrote: > > On Mon, 26 May 2003, Jens Axboe wrote: > > > > > Think of all the fairness issues we've had in the elevator code, and > > > realize that the low-level disk probably implements _none_ of those > > > fairness algorithms. > > > > I think it does, to some extent at least. > > I doubt they do a very good job of it. I know of bad cases, even with > "high-end" hardware. Sure, we can hope that it's getting better, but do we > want to bet on it.
No I agree, it's always nice to handle these cases in software so we don't have to rely on these things getting fixed.
> > > Hmm.. Where does it keep track of request latency for requests that have > > > been removed from the queue? > > > > Well, it doesn't... > > Yeah. Which means that right now _really_ long starvation will show up as > timeouts, while other cases will just show up as bad latency. > > Which will _work_, of course (assuming the timeout handling is correct, > which is a big if in itself), but it still sucks from a usability > standpoint. > > Even if we drop our timeouts from 30 seconds (or whatever they are now) > down to just a few seconds, that's a _loooong_ time, and we should be a > lot more proactive about things. Audio/video stuff tends to want things > with latencies in the tenth-of-a-second range, even when they buffer > things up internally to hide the worst cases.
Here's something ridicolously simple, that just wont start a new tag if the oldest tag is older than 100ms. Clearly nothing for submission, but it gets the point across.
Now only look at reads, and we've got something a little useful at least.
James, speaking of queue localities and tcq... Doug mentioned some time ago that aic7xxx dishes out tags numbers from a hba pool which makes it impossible to support with out current block layer queueing code. Maybe it we associate the blk_queue_tag structure with a bunch of queues instead of having a 1:1 mapping it could work.
===== drivers/block/ll_rw_blk.c 1.170 vs edited ===== --- 1.170/drivers/block/ll_rw_blk.c Thu May 8 11:30:11 2003 +++ edited/drivers/block/ll_rw_blk.c Tue May 27 08:44:44 2003 @@ -574,6 +574,13 @@ BUG(); } + if (!list_empty(&bqt->busy_list)) { + struct request *__rq = list_entry_rq(bqt->busy_list.prev); + + if (time_after(rq->timeout, jiffies)) + return 1; + } + for (map = bqt->tag_map; *map == -1UL; map++) { tag += BLK_TAGS_PER_LONG; @@ -589,6 +596,7 @@ bqt->tag_index[tag] = rq; blkdev_dequeue_request(rq); list_add(&rq->queuelist, &bqt->busy_list); + rq->timeout = jiffies + HZ / 10; bqt->busy++; return 0; } -- Jens Axboe
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