Messages in this thread | | | Date | Sun, 24 Aug 2008 21:24:37 +0100 | From | "Daniel J Blueman" <> | Subject | Re: performance "regression" in cfq compared to anticipatory, deadline and noop |
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Hi Fabio, Jens,
On Thu, May 15, 2008 at 1:21 PM, Fabio Checconi <fchecconi@gmail.com> wrote: >> From: Jens Axboe <jens.axboe@oracle.com> >> Date: Thu, May 15, 2008 09:01:28AM +0200 >> >> I don't think it's 2.6.25 vs 2.6.26-rc2, I can still reproduce some >> request size offsets with the patch. So still fumbling around with this, >> I'll be sending out another test patch when I'm confident it's solved >> the size issue. > > IMO an interesting thing is how/why anticipatory doesn't show the > issue. The device is not put into ANTIC_WAIT_NEXT if there is no > dispatch returning no requests while the queue is not empty. This > seems to be enough in the reported workloads. > > I don't think this behavior is the correct one (it is still racy > WRT merges after breaking anticipation) anyway it should make things > a little bit better. I fear that a complete solution would not > involve only the scheduler. > > Introducing the very same behavior in cfq seems to be not so easy > (i.e., start idling only if there was a dispatch round while the > last request was being served) but an approximated version can be > introduced quite easily. The patch below should do that, rescheduling > the dispatch only if necessary; it is not tested at all, just posted > for discussion. > > --- > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index b399c62..41f1e0e 100644 > --- a/block/cfq-iosched.c > +++ b/block/cfq-iosched.c > @@ -169,6 +169,7 @@ enum cfqq_state_flags { > CFQ_CFQQ_FLAG_queue_new, /* queue never been serviced */ > CFQ_CFQQ_FLAG_slice_new, /* no requests dispatched in slice */ > CFQ_CFQQ_FLAG_sync, /* synchronous queue */ > + CFQ_CFQQ_FLAG_dispatched, /* empty dispatch while idling */ > }; > > #define CFQ_CFQQ_FNS(name) \ > @@ -196,6 +197,7 @@ CFQ_CFQQ_FNS(prio_changed); > CFQ_CFQQ_FNS(queue_new); > CFQ_CFQQ_FNS(slice_new); > CFQ_CFQQ_FNS(sync); > +CFQ_CFQQ_FNS(dispatched); > #undef CFQ_CFQQ_FNS > > static void cfq_dispatch_insert(struct request_queue *, struct request *); > @@ -749,6 +751,7 @@ static void __cfq_set_active_queue(struct cfq_data *cfqd, > cfqq->slice_end = 0; > cfq_clear_cfqq_must_alloc_slice(cfqq); > cfq_clear_cfqq_fifo_expire(cfqq); > + cfq_clear_cfqq_dispatched(cfqq); > cfq_mark_cfqq_slice_new(cfqq); > cfq_clear_cfqq_queue_new(cfqq); > } > @@ -978,6 +981,7 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd) > */ > if (timer_pending(&cfqd->idle_slice_timer) || > (cfqq->dispatched && cfq_cfqq_idle_window(cfqq))) { > + cfq_mark_cfqq_dispatched(cfqq); > cfqq = NULL; > goto keep_queue; > } > @@ -1784,7 +1788,10 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq, > if (cfq_cfqq_wait_request(cfqq)) { > cfq_mark_cfqq_must_dispatch(cfqq); > del_timer(&cfqd->idle_slice_timer); > - blk_start_queueing(cfqd->queue); > + if (cfq_cfqq_dispatched(cfqq)) { > + cfq_clear_cfqq_dispatched(cfqq); > + cfq_schedule_dispatch(cfqd); > + } > } > } else if (cfq_should_preempt(cfqd, cfqq, rq)) { > /*
This was the last test I didn't get around to. Alas, is did help, but didn't give the merging required for full performance:
# echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null bs=128k count=2000 262144000 bytes (262 MB) copied, 2.47787 s, 106 MB/s
# echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda Timing buffered disk reads: 308 MB in 3.01 seconds = 102.46 MB/sec
It is an improvement over the baseline performance of 2.6.27-rc4:
# echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null bs=128k count=2000 262144000 bytes (262 MB) copied, 2.56514 s, 102 MB/s
# echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda Timing buffered disk reads: 294 MB in 3.02 seconds = 97.33 MB/sec
Note that platter speed is around 125MB/s (which I get near at smaller read sizes).
I feel 128KB read requests are perhaps important, as this is a commonly-used RAID stripe size, and may explain the read-performance drop sometimes we see in hardware vs software RAID benchmarks.
How can we generate some ideas or movement on fixing/improving this behaviour?
Thanks! Daniel -- Daniel J Blueman
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