Re: Switching to MQ by default may generate some bug reports

Paolo Valente
2017-08-08T08:06:09Z

> Il giorno 07 ago 2017, alle ore 20:42, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>
>>
>> Il giorno 07 ago 2017, alle ore 19:32, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>
>>>
>>> Il giorno 05 ago 2017, alle ore 00:05, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>>
>>>>
>>>> Il giorno 04 ago 2017, alle ore 13:01, Mel Gorman <mgorman@techsingularity.net> ha scritto:
>>>>
>>>> On Fri, Aug 04, 2017 at 09:26:20AM +0200, Paolo Valente wrote:
>>>>>> I took that into account BFQ with low-latency was also tested and the
>>>>>> impact was not a universal improvement although it can be a noticable
>>>>>> improvement. From the same machine;
>>>>>>
>>>>>> dbench4 Loadfile Execution Time
>>>>>> 4.12.0 4.12.0 4.12.0
>>>>>> legacy-cfq mq-bfq mq-bfq-tput
>>>>>> Amean 1 80.67 ( 0.00%) 83.68 ( -3.74%) 84.70 ( -5.00%)
>>>>>> Amean 2 92.87 ( 0.00%) 121.63 ( -30.96%) 88.74 ( 4.45%)
>>>>>> Amean 4 102.72 ( 0.00%) 474.33 (-361.77%) 113.97 ( -10.95%)
>>>>>> Amean 32 2543.93 ( 0.00%) 1927.65 ( 24.23%) 2038.74 ( 19.86%)
>>>>>>
>>>>>
>>>>> Thanks for trying with low_latency disabled. If I read numbers
>>>>> correctly, we move from a worst case of 361% higher execution time to
>>>>> a worst case of 11%. With a best case of 20% of lower execution time.
>>>>>
>>>>
>>>> Yes.
>>>>
>>>>> I asked you about none and mq-deadline in a previous email, because
>>>>> actually we have a double change here: change of the I/O stack, and
>>>>> change of the scheduler, with the first change probably not irrelevant
>>>>> with respect to the second one.
>>>>>
>>>>
>>>> True. However, the difference between legacy-deadline mq-deadline is
>>>> roughly around the 5-10% mark across workloads for SSD. It's not
>>>> universally true but the impact is not as severe. While this is not
>>>> proof that the stack change is the sole root cause, it makes it less
>>>> likely.
>>>>
>>>
>>> I'm getting a little lost here. If I'm not mistaken, you are saying,
>>> since the difference between two virtually identical schedulers
>>> (legacy-deadline and mq-deadline) is only around 5-10%, while the
>>> difference between cfq and mq-bfq-tput is higher, then in the latter
>>> case it is not the stack's fault. Yet the loss of mq-bfq-tput in the
>>> above test is exactly in the 5-10% range? What am I missing? Other
>>> tests with mq-bfq-tput not yet reported?
>>>
>>>>> By chance, according to what you have measured so far, is there any
>>>>> test where, instead, you expect or have seen bfq-mq-tput to always
>>>>> lose? I could start from there.
>>>>>
>>>>
>>>> global-dhp__io-fio-randread-async-randwrite-xfs but marginal enough that
>>>> it could be the stack change.
>>>>
>>>> global-dhp__io-dbench4-fsync-ext4 was a universal loss across any
>>>> machine tested. This is global-dhp__io-dbench4-fsync from mmtests using
>>>> ext4 as a filesystem. The same is not true for XFS so the filesystem
>>>> matters.
>>>>
>>>
>>> Ok, then I will try to repeat global-dhp__io-dbench4-fsync-ext4 as
>>> soon as I can, thanks.
>>>
>>>
>>
>> I've run this test and tried to further investigate this regression.
>> For the moment, the gist seems to be that blk-mq plays an important
>> role, not only with bfq (unless I'm considering the wrong numbers).
>> Even if your main purpose in this thread was just to give a heads-up,
>> I guess it may be useful to share what I have found out. In addition,
>> I want to ask for some help, to try to get closer to the possible
>> causes of at least this regression. If you think it would be better
>> to open a new thread on this stuff, I'll do it.
>>
>> First, I got mixed results on my system. I'll focus only on the the
>> case where mq-bfq-tput achieves its worst relative performance w.r.t.
>> to cfq, which happens with 64 clients. Still, also in this case
>> mq-bfq is better than cfq in all average values, but Flush. I don't
>> know which are the best/right values to look at, so, here's the final
>> report for both schedulers:
>>
>> CFQ
>>
>> Operation Count AvgLat MaxLat
>> --------------------------------------------------
>> Flush 13120 20.069 348.594
>> Close 133696 0.008 14.642
>> LockX 512 0.009 0.059
>> Rename 7552 1.857 415.418
>> ReadX 270720 0.141 535.632
>> WriteX 89591 421.961 6363.271
>> Unlink 34048 1.281 662.467
>> UnlockX 512 0.007 0.057
>> FIND_FIRST 62016 0.086 25.060
>> SET_FILE_INFORMATION 15616 0.995 176.621
>> QUERY_FILE_INFORMATION 28734 0.004 1.372
>> QUERY_PATH_INFORMATION 170240 0.163 820.292
>> QUERY_FS_INFORMATION 28736 0.017 4.110
>> NTCreateX 178688 0.437 905.567
>>
>> MQ-BFQ-TPUT
>>
>> Operation Count AvgLat MaxLat
>> --------------------------------------------------
>> Flush 13504 75.828 11196.035
>> Close 136896 0.004 3.855
>> LockX 640 0.005 0.031
>> Rename 8064 1.020 288.989
>> ReadX 297600 0.081 685.850
>> WriteX 93515 391.637 12681.517
>> Unlink 34880 0.500 146.928
>> UnlockX 640 0.004 0.032
>> FIND_FIRST 63680 0.045 222.491
>> SET_FILE_INFORMATION 16000 0.436 686.115
>> QUERY_FILE_INFORMATION 30464 0.003 0.773
>> QUERY_PATH_INFORMATION 175552 0.044 148.449
>> QUERY_FS_INFORMATION 29888 0.009 1.984
>> NTCreateX 183152 0.289 300.867
>>
>> Are these results in line with yours for this test?
>>
>> Anyway, to investigate this regression more in depth, I took two
>> further steps. First, I repeated the same test with bfq-sq, my
>> out-of-tree version of bfq for legacy block (identical to mq-bfq apart
>> from the changes needed for bfq to live in blk-mq). I got:
>>
>> BFQ-SQ-TPUT
>>
>> Operation Count AvgLat MaxLat
>> --------------------------------------------------
>> Flush 12618 30.212 484.099
>> Close 123884 0.008 10.477
>> LockX 512 0.010 0.170
>> Rename 7296 2.032 426.409
>> ReadX 262179 0.251 985.478
>> WriteX 84072 461.398 7283.003
>> Unlink 33076 1.685 848.734
>> UnlockX 512 0.007 0.036
>> FIND_FIRST 58690 0.096 220.720
>> SET_FILE_INFORMATION 14976 1.792 466.435
>> QUERY_FILE_INFORMATION 26575 0.004 2.194
>> QUERY_PATH_INFORMATION 158125 0.112 614.063
>> QUERY_FS_INFORMATION 28224 0.017 1.385
>> NTCreateX 167877 0.827 945.644
>>
>> So, the worst-case regression is now around 15%. This made me suspect
>> that blk-mq influences results a lot for this test. To crosscheck, I
>> compared legacy-deadline and mq-deadline too.
>>
>
> Ok, found the problem for the 15% loss in bfq-sq. bfq-sq gets
> occasionally confused by the workload, and grants device idling to
> processes that, for this specific workload, would be better to
> de-schedule immediately. If we set slice_idle to 0, then bfq-sq
> becomes more or less equivalent to cfq (for some operations apparently
> even much better):
>
> bfq-sq-tput-0idle
>
> Operation Count AvgLat MaxLat
> --------------------------------------------------
> Flush 13013 17.888 280.517
> Close 133004 0.008 20.698
> LockX 512 0.008 0.088
> Rename 7427 2.041 193.232
> ReadX 270534 0.138 408.534
> WriteX 88598 429.615 6272.212
> Unlink 33734 1.205 559.152
> UnlockX 512 0.011 1.808
> FIND_FIRST 61762 0.087 23.012
> SET_FILE_INFORMATION 15337 1.322 220.155
> QUERY_FILE_INFORMATION 28415 0.004 0.559
> QUERY_PATH_INFORMATION 169423 0.150 580.570
> QUERY_FS_INFORMATION 28547 0.019 24.466
> NTCreateX 177618 0.544 681.795
>
> I'll try soon with mq-bfq too, for which I expect however a deeper
> investigation to be needed.
>

Hi,
to test mq-bfq (with both slice_idle==0 and slice_idle>0), I have also
applied Ming patches, and Ah, victory!

Regardless of the value of slice idle:

mq-bfq-tput

Operation Count AvgLat MaxLat
--------------------------------------------------
Flush 13183 70.381 1025.407
Close 134539 0.004 1.011
LockX 512 0.005 0.025
Rename 7721 0.740 404.979
ReadX 274422 0.126 873.364
WriteX 90535 408.371 7400.585
Unlink 34276 0.634 581.067
UnlockX 512 0.003 0.029
FIND_FIRST 62664 0.052 321.027
SET_FILE_INFORMATION 15981 0.234 124.739
QUERY_FILE_INFORMATION 29042 0.003 1.731
QUERY_PATH_INFORMATION 171769 0.032 522.415
QUERY_FS_INFORMATION 28958 0.009 3.043
NTCreateX 179643 0.298 687.466

Throughput 9.11183 MB/sec 64 clients 64 procs max_latency=7400.588 ms

Differently from bfq-sq, setting slice_idle to 0 doesn't provide any
benefit, which lets me suspect that there is some other issue in
blk-mq (only a suspect). I think I may have already understood how to
guarantee that bfq almost never idles the device uselessly also for
this workload. Yet, since in blk-mq there is no gain even after
excluding useless idling, I'll wait for at least Ming's patches to be
merged before possibly proposing this contribution. Maybe some other
little issue related to this lack of gain in blk-mq will be found and
solved in the meantime.

Moving to the read-write unfairness problem.

Thanks,
Paolo

> Thanks,
> Paolo
>
>> LEGACY-DEADLINE
>>
>> Operation Count AvgLat MaxLat
>> --------------------------------------------------
>> Flush 13267 9.622 298.206
>> Close 135692 0.007 10.627
>> LockX 640 0.008 0.066
>> Rename 7827 0.544 481.123
>> ReadX 285929 0.220 2698.442
>> WriteX 92309 430.867 5191.608
>> Unlink 34534 1.133 619.235
>> UnlockX 640 0.008 0.724
>> FIND_FIRST 63289 0.086 56.851
>> SET_FILE_INFORMATION 16000 1.254 844.065
>> QUERY_FILE_INFORMATION 29883 0.004 0.618
>> QUERY_PATH_INFORMATION 173232 0.089 1295.651
>> QUERY_FS_INFORMATION 29632 0.017 4.813
>> NTCreateX 181464 0.479 2214.343
>>
>>
>> MQ-DEADLINE
>>
>> Operation Count AvgLat MaxLat
>> --------------------------------------------------
>> Flush 13760 90.542 13221.495
>> Close 137654 0.008 27.133
>> LockX 640 0.009 0.115
>> Rename 8064 1.062 246.759
>> ReadX 297956 0.051 347.018
>> WriteX 94698 425.636 15090.020
>> Unlink 35077 0.580 208.462
>> UnlockX 640 0.007 0.291
>> FIND_FIRST 66630 0.566 530.339
>> SET_FILE_INFORMATION 16000 1.419 811.494
>> QUERY_FILE_INFORMATION 30717 0.004 1.108
>> QUERY_PATH_INFORMATION 176153 0.182 517.419
>> QUERY_FS_INFORMATION 30857 0.018 18.562
>> NTCreateX 184145 0.281 582.076
>>
>> So, with both bfq and deadline there seems to be a serious regression,
>> especially on MaxLat, when moving from legacy block to blk-mq. The
>> regression is much worse with deadline, as legacy-deadline has the
>> lowest max latency among all the schedulers, whereas mq-deadline has
>> the highest one.
>>
>> Regardless of the actual culprit of this regression, I would like to
>> investigate further this issue. In this respect, I would like to ask
>> for a little help. I would like to isolate the workloads generating
>> the highest latencies. To this purpose, I had a look at the loadfile
>> client-tiny.txt, and I still have a doubt: is every item in the
>> loadfile executed somehow several times (for each value of the number
>> of clients), or is it executed only once? More precisely, IIUC, for
>> each operation reported in the above results, there are several items
>> (lines) in the loadfile. So, is each of these items executed only
>> once?
>>
>> I'm asking because, if it is executed only once, then I guess I can
>> find the critical tasks ore easily. Finally, if it is actually
>> executed only once, is it expected that the latency for such a task is
>> one order of magnitude higher than that of the average latency for
>> that group of tasks? I mean, is such a task intrinsically much
>> heavier, and then expectedly much longer, or is the fact that latency
>> is much higher for this task a sign that something in the kernel
>> misbehaves for that task?
>>
>> While waiting for some feedback, I'm going to execute your test
>> showing great unfairness between writes and reads, and to also check
>> whether responsiveness does worsen if the write workload for that test
>> is being executed in the background.
>>
>> Thanks,
>> Paolo
>>
>>> ...
>>>> --
>>>> Mel Gorman
>>>> SUSE Labs