Messages in this thread Patch in this message | | | Date | Wed, 28 Apr 2010 16:02:22 -0400 | From | Vivek Goyal <> | Subject | Re: CFQ read performance regression |
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On Tue, Apr 27, 2010 at 07:25:14PM +0200, Corrado Zoccolo wrote: > On Mon, Apr 26, 2010 at 9:14 PM, Vivek Goyal <vgoyal@redhat.com> wrote: > > On Sat, Apr 24, 2010 at 10:36:48PM +0200, Corrado Zoccolo wrote: > > > > [..] > >> >> Anyway, if that's the case, then we probably need to allow IO from > >> >> multiple sequential readers and keep a watch on throughput. If throughput > >> >> drops then reduce the number of parallel sequential readers. Not sure how > >> >> much of code that is but with multiple cfqq going in parallel, ioprio > >> >> logic will more or less stop working in CFQ (on multi-spindle hardware). > >> Hi Vivek, > >> I tried to implement exactly what you are proposing, see the attached patches. > >> I leverage the queue merging features to let multiple cfqqs share the > >> disk in the same timeslice. > >> I changed the queue split code to trigger on throughput drop instead > >> of on seeky pattern, so diverging queues can remain merged if they > >> have good throughput. Moreover, I measure the max bandwidth reached by > >> single queues and merged queues (you can see the values in the > >> bandwidth sysfs file). > >> If merged queues can outperform non-merged ones, the queue merging > >> code will try to opportunistically merge together queues that cannot > >> submit enough requests to fill half of the NCQ slots. I'd like to know > >> if you can see any improvements out of this on your hardware. There > >> are some magic numbers in the code, you may want to try tuning them. > >> Note that, since the opportunistic queue merging will start happening > >> only after merged queues have shown to reach higher bandwidth than > >> non-merged queues, you should use the disk for a while before trying > >> the test (and you can check sysfs), or the merging will not happen. > > > > Hi Corrado, > > > > I ran these patches and I did not see any improvement. I think the reason > > being that no cooperative queue merging took place and we did not have > > any data for throughput with coop flag on. > > > > #cat /sys/block/dm-3/queue/iosched/bandwidth > > 230 753 0 > > > > I think we need to implement something similiar to hw_tag detection logic > > where we allow dispatches from multiple sync-idle queues at a time and try > > to observe the BW. After certain window once we have observed the window, > > then set the system behavior accordingly. > Hi Vivek, > thanks for testing. Can you try changing the condition to enable the > queue merging to also consider the case in which max_bw[1] == 0 && > max_bw[0] > 100MB/s (notice that max_bw is measured in > sectors/jiffie). > This should rule out low end disks, and enable merging where it can be > beneficial. > If the results are good, but we find this enabling condition > unreliable, then we can think of a better way, but I'm curious to see > if the results are promising before thinking to the details.
Ok, I made some changes and now some queue merging seems to be happening and I am getting little better BW. This will require more debugging. I will try to spare some time later.
Kernel=2.6.34-rc5-corrado-multicfq DIR= /mnt/iostmnt/fio DEV= /dev/mapper/mpathe Workload=bsr iosched=cfq Filesz=1G bs=16K ========================================================================== job Set NR ReadBW(KB/s) MaxClat(us) WriteBW(KB/s) MaxClat(us) --- --- -- ------------ ----------- ------------- ----------- bsr 1 1 136457 67353 0 0 bsr 1 2 148008 192415 0 0 bsr 1 4 180223 535205 0 0 bsr 1 8 166983 542326 0 0 bsr 1 16 176617 832188 0 0
Output of iosched/bandwidth
0 546 740
I did following changes on top of your patch.
Vivek
--- block/cfq-iosched.c | 11 +++++++++-- 1 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c index 4e9e015..7589c44 100644 --- a/block/cfq-iosched.c +++ b/block/cfq-iosched.c @@ -243,6 +243,7 @@ struct cfq_data { */ int hw_tag_est_depth; unsigned int hw_tag_samples; + unsigned int cfqq_merged_samples; /* * performance measurements * max_bw is indexed by coop flag. @@ -1736,10 +1737,14 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd, // Opportunistic queue merging could be beneficial even on far queues // We enable it only on NCQ disks, if we observed that merged queues // can reach higher bandwidth than single queues. + // 204 sectors per jiffy is equivalent to 100MB/s on 1000 HZ conf. + // Allow merge if we don't have sufficient merged cfqq samples. rs = cur_cfqq->allocated[READ] + cur_cfqq->allocated[WRITE]; - if (cfqd->hw_tag && cfqd->max_bw[1] > cfqd->max_bw[0] && + if (cfqd->hw_tag + && (cfqd->max_bw[1] > cfqd->max_bw[0] + || (cfqd->max_bw[0] > 204 && !sample_valid(cfqd->cfqq_merged_samples))) // Do not overload the device queue - rs < cfqd->hw_tag_est_depth / 2) { + && rs < cfqd->hw_tag_est_depth / 2) { unsigned r1 = __cfqq->allocated[READ] + __cfqq->allocated[WRITE]; unsigned r2 = __cfqq1->allocated[READ] + __cfqq1->allocated[WRITE]; // Prefer merging with a queue that has fewer pending requests @@ -1750,6 +1755,8 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd, // Do not merge if the merged queue would have too many requests if (r1 + rs > cfqd->hw_tag_est_depth / 2) return NULL; + cfqd->cfqq_merged_samples++; + // Merge only if the BW of the two queues is comparable if (abs(__cfqq->last_bw - cur_cfqq->last_bw) * 4 < cfqd->max_bw[0]) return __cfqq; -- 1.6.2.5 > > Thanks, > Corrado > > > > > Kernel=2.6.34-rc5-corrado-multicfq > > DIR= /mnt/iostmnt/fio DEV= /dev/mapper/mpathe > > Workload=bsr iosched=cfq Filesz=2G bs=4K > > ========================================================================== > > job Set NR ReadBW(KB/s) MaxClat(us) WriteBW(KB/s) MaxClat(us) > > --- --- -- ------------ ----------- ------------- ----------- > > bsr 1 1 126590 61448 0 0 > > bsr 1 2 127849 242843 0 0 > > bsr 1 4 131886 508021 0 0 > > bsr 1 8 131890 398241 0 0 > > bsr 1 16 129167 454244 0 0 > > > > Thanks > > Vivek > > > > > > -- > __________________________________________________________________________ > > dott. Corrado Zoccolo mailto:czoccolo@gmail.com > PhD - Department of Computer Science - University of Pisa, Italy > -------------------------------------------------------------------------- > The self-confidence of a warrior is not the self-confidence of the average > man. The average man seeks certainty in the eyes of the onlooker and calls > that self-confidence. The warrior seeks impeccability in his own eyes and > calls that humbleness. > Tales of Power - C. Castaneda -- 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/
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