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SubjectRe: performance "regression" in cfq compared to anticipatory, deadline and noop
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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