Messages in this thread | | | Date | Wed, 2 Dec 2009 10:02:34 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH 4/4] cfq-iosched: fix corner cases in idling logic |
| |
On Wed, Dec 02, 2009 at 09:47:59AM -0500, Jeff Moyer wrote: > Vivek Goyal <vgoyal@redhat.com> writes: > > > On Wed, Dec 02, 2009 at 03:14:22PM +0100, Corrado Zoccolo wrote: > >> Hi Jeff, > >> On Wed, Dec 2, 2009 at 2:42 PM, Jeff Moyer <jmoyer@redhat.com> wrote: > >> > Corrado Zoccolo <czoccolo@gmail.com> writes: > >> > > >> >> Idling logic was disabled in some corner cases, leading to unfair share > >> >> for noidle queues. > >> >> * the idle timer was not armed if there were other requests in the > >> >> driver. unfortunately, those requests could come from other workloads, > >> >> or queues for which we don't enable idling. So we will check only > >> >> pending requests from the active queue > >> >> * rq_noidle check on no-idle queue could disable the end of tree idle if > >> >> the last completed request was rq_noidle. Now, we will disable that > >> >> idle only if all the queues served in the no-idle tree had rq_noidle > >> >> requests. > >> >> > >> >> Reported-by: Vivek Goyal <vgoyal@redhat.com> > >> >> Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com> > >> > > >> >> @@ -2606,17 +2608,27 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq) > >> >> cfq_clear_cfqq_slice_new(cfqq); > >> >> } > >> >> /* > >> >> - * If there are no requests waiting in this queue, and > >> >> - * there are other queues ready to issue requests, AND > >> >> - * those other queues are issuing requests within our > >> >> - * mean seek distance, give them a chance to run instead > >> >> - * of idling. > >> >> + * Idling is not enabled on: > >> >> + * - expired queues > >> >> + * - idle-priority queues > >> >> + * - async queues > >> >> + * - queues with still some requests queued > >> >> + * - when there is a close cooperator > >> >> */ > >> > > >> > I'm not sure this logic is correct. Is this for the 2.6.33 branch? > >> Yes. > >> > If so, the coop flag now means that multiple processes share the same > >> > cfqq. Are you sure this is the right thing to do for close cooperators? > >> I'm not sure. I didn't change the logic for close cooperators: > > Heh, right you are. > > >> - else if (cfqq_empty && !cfq_close_cooperator(cfqd, cfqq) && > >> - sync && !rq_noidle(rq)) > >> - cfq_arm_slice_timer(cfqd); > >> + else if (sync && cfqq_empty && > >> + !cfq_close_cooperator(cfqd, cfqq)) { > >> + cfqd->noidle_tree_requires_idle |= !rq_noidle(rq); > >> > >> I changed the rq_noidle part, and rewrote the comment to be aligned > >> with the code. > >> So I don't mind if you improve (or just remove) the close cooperator part. > >> Probably, you should do a test where close cooperating processes are competing > >> with a sequential reader, to see the effect of idling or not on them. > >> > > > > I also can't find what's wrong with this. Previously we were not merging > > close cooperators in a single queue. So if we found a close cooperator > > we chose to not idle and move to that close cooperator. Now we try to > > merge all the close cooperators in a single queue. But that merging has > > not taken place yet and will happen when next request comes. > > The coop flag is not set until the merge has taken place. > > > A normal sequential reader will not find the close cooperator. Only the > > queues which should be merged will find the close cooperator. If anyway > > these queues are going to be merged soon, there is proably no point in > > continuing to idle on this queue in case we found a close cooperator. > > > > So, to me even in new code by jeff, it probably is fine to continue with > > policy of not idling if we found a close cooperator. > > That would mean changing the check from cfqq_coop to cfqq->new_queue != > NULL.
Does it make a big difference. cfq_close_cooperator() does not seem to be relying on coop flag. It will return us a queue if it thinks there is a close cooperator. (Irrespective of the fact whether cfqq->new_cfqq has bee setup yet or not). IIUC, cfqq->new_cfqq will be set in select_queue(). So in case select_queue() has not run yet, then cfqq->new_cfqq = NULL but we have a close cooperator.
But I guess this condition will not hit many a times as select_queue() happens very frequently on NCQ hardware and the moment select queue finds close cooperator it will expire the current queue and above check will not even get a chance to turn.
So IIUC, if we are here cfqq->new_cfqq is always NULL otherwise select_queue() by now must have expired us and we will not be here. So either we can completely remove the check or we can just continue with above check.
Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |