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Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.8.0 Subject: Re: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load Content-Language: en-US To: "yukuai (C)" , axboe@kernel.dk, bvanassche@acm.org, andriy.shevchenko@linux.intel.com, john.garry@huawei.com, ming.lei@redhat.com, qiulaibin@huawei.com Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com References: <20220415101053.554495-1-yukuai3@huawei.com> <3fbadd9f-11dd-9043-11cf-f0839dcf30e1@opensource.wdc.com> <63e84f2a-2487-a0c3-cab2-7d2011bc2db4@huawei.com> From: Damien Le Moal Organization: Western Digital Research In-Reply-To: <63e84f2a-2487-a0c3-cab2-7d2011bc2db4@huawei.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 4/25/22 15:14, yukuai (C) wrote: > =E5=9C=A8 2022/04/25 11:24, Damien Le Moal =E5=86=99=E9=81=93: >> On 4/24/22 11:43, yukuai (C) wrote: >>> friendly ping ... >>> >>> =E5=9C=A8 2022/04/15 18:10, Yu Kuai =E5=86=99=E9=81=93: >>>> Changes in v3: >>>> - update 'waiters_cnt' before 'ws_active' in sbitmap_prepare_to_w= ait() >>>> in patch 1, in case __sbq_wake_up() see 'ws_active > 0' while >>>> 'waiters_cnt' are all 0, which will cause deap loop. >>>> - don't add 'wait_index' during each loop in patch 2 >>>> - fix that 'wake_index' might mismatch in the first wake up in pa= tch 3, >>>> also improving coding for the patch. >>>> - add a detection in patch 4 in case io hung is triggered in corn= er >>>> cases. >>>> - make the detection, free tags are sufficient, more flexible. >>>> - fix a race in patch 8. >>>> - fix some words and add some comments. >>>> >>>> Changes in v2: >>>> - use a new title >>>> - add patches to fix waitqueues' unfairness - path 1-3 >>>> - delete patch to add queue flag >>>> - delete patch to split big io thoroughly >>>> >>>> In this patchset: >>>> - patch 1-3 fix waitqueues' unfairness. >>>> - patch 4,5 disable tag preemption on heavy load. >>>> - patch 6 forces tag preemption for split bios. >>>> - patch 7,8 improve large random io for HDD. We do meet the probl= em and >>>> I'm trying to fix it at very low cost. However, if anyone still t= hinks >>>> this is not a common case and not worth to optimize, I'll drop th= em. >>>> >>>> There is a defect for blk-mq compare to blk-sq, specifically split i= o >>>> will end up discontinuous if the device is under high io pressure, w= hile >>>> split io will still be continuous in sq, this is because: >>>> >>>> 1) new io can preempt tag even if there are lots of threads waiting. >>>> 2) split bio is issued one by one, if one bio can't get tag, it will= go >>>> to wail. >>>> 3) each time 8(or wake batch) requests is done, 8 waiters will be wo= ken up. >>>> Thus if a thread is woken up, it will unlikey to get multiple tags. >>>> >>>> The problem was first found by upgrading kernel from v3.10 to v4.18, >>>> test device is HDD with 256 'max_sectors_kb', and test case is issui= ng 1m >>>> ios with high concurrency. >>>> >>>> Noted that there is a precondition for such performance problem: >>>> There is a certain gap between bandwidth for single io with >>>> bs=3Dmax_sectors_kb and disk upper limit. >>>> >>>> During the test, I found that waitqueues can be extremly unbalanced = on >>>> heavy load. This is because 'wake_index' is not set properly in >>>> __sbq_wake_up(), see details in patch 3. >>>> >>>> Test environment: >>>> arm64, 96 core with 200 BogoMIPS, test device is HDD. The default >>>> 'max_sectors_kb' is 1280(Sorry that I was unable to test on the mach= ine >>>> where 'max_sectors_kb' is 256).>> >>>> The single io performance(randwrite): >>>> >>>> | bs | 128k | 256k | 512k | 1m | 1280k | 2m | 4m | >>>> | -------- | ---- | ---- | ---- | ---- | ----- | ---- | ---- | >>>> | bw MiB/s | 20.1 | 33.4 | 51.8 | 67.1 | 74.7 | 82.9 | 82.9 | >> >> These results are extremely strange, unless you are running with the >> device write cache disabled ? If you have the device write cache enabl= ed, >> the problem you mention above would be most likely completely invisibl= e, >> which I guess is why nobody really noticed any issue until now. >> >> Similarly, with reads, the device side read-ahead may hide the problem= , >> albeit that depends on how "intelligent" the drive is at identifying >> sequential accesses. >> >>>> >>>> It can be seen that 1280k io is already close to upper limit, and it= 'll >>>> be hard to see differences with the default value, thus I set >>>> 'max_sectors_kb' to 128 in the following test. >>>> >>>> Test cmd: >>>> fio \ >>>> -filename=3D/dev/$dev \ >>>> -name=3Dtest \ >>>> -ioengine=3Dpsync \ >>>> -allow_mounted_write=3D0 \ >>>> -group_reporting \ >>>> -direct=3D1 \ >>>> -offset_increment=3D1g \ >>>> -rw=3Drandwrite \ >>>> -bs=3D1024k \ >>>> -numjobs=3D{1,2,4,8,16,32,64,128,256,512} \ >>>> -runtime=3D110 \ >>>> -ramp_time=3D10 >>>> >>>> Test result: MiB/s >>>> >>>> | numjobs | v5.18-rc1 | v5.18-rc1-patched | >>>> | ------- | --------- | ----------------- | >>>> | 1 | 67.7 | 67.7 | >>>> | 2 | 67.7 | 67.7 | >>>> | 4 | 67.7 | 67.7 | >>>> | 8 | 67.7 | 67.7 | >>>> | 16 | 64.8 | 65.6 | >>>> | 32 | 59.8 | 63.8 | >>>> | 64 | 54.9 | 59.4 | >>>> | 128 | 49 | 56.9 | >>>> | 256 | 37.7 | 58.3 | >>>> | 512 | 31.8 | 57.9 | >> >> Device write cache disabled ? >> >> Also, what is the max QD of this disk ? >> >> E.g., if it is SATA, it is 32, so you will only get at most 64 schedul= er >> tags. So for any of your tests with more than 64 threads, many of the >> threads will be waiting for a scheduler tag for the BIO before the >> bio_split problem you explain triggers. Given that the numbers you sho= w >> are the same for before-after patch with a number of threads <=3D 64, = I am >> tempted to think that the problem is not really BIO splitting... >> >> What about random read workloads ? What kind of results do you see ? >=20 > Hi, >=20 > Sorry about the misleading of this test case. >=20 > This testcase is high concurrency huge randwrite, it's just for the > problem that split bios won't be issued continuously, which is the > root cause of the performance degradation as the numjobs increases. >=20 > queue_depth is 32, and numjobs is 64, thus when numjobs is not greater > than 8, performance is fine, because the ratio of sequential io should > be 7/8. However, as numjobs increases, performance is worse because > the ratio is lower. For example, when numjobs is 512, the ratio of > sequential io is about 20%. But with 512 jobs, you will get only 64 jobs only with IOs in the queue. All other jobs will be waiting for a scheduler tag before being able to issue their large BIO. No ? It sounds like the set of scheduler tags should be a bit more elastic: always allow BIOs from a split of a large BIO to be submitted (that is to get a scheduler tag) even if that causes a temporary excess of the number of requests beyond the default number of scheduler tags. Doing so, all fragments of a large BIOs can be queued immediately. From there, if the scheduler operates correctly, all the requests from the large BIOs split would be issued in sequence to the device. >=20 > patch 6-8 will let split bios still be issued continuously under high > pressure. >=20 > Thanks, > Kuai >=20 --=20 Damien Le Moal Western Digital Research