On 4/25/22 15:14, yukuai (C) wrote:
> 在 2022/04/25 11:24, Damien Le Moal 写道:
>> On 4/24/22 11:43, yukuai (C) wrote:
>>> friendly ping ...
>>>
>>> 在 2022/04/15 18:10, Yu Kuai 写道:
>>>> Changes in v3:
>>>> - update 'waiters_cnt' before 'ws_active' in sbitmap_prepare_to_wait()
>>>> 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 patch 3,
>>>> also improving coding for the patch.
>>>> - add a detection in patch 4 in case io hung is triggered in corner
>>>> 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 problem and
>>>> I'm trying to fix it at very low cost. However, if anyone still thinks
>>>> this is not a common case and not worth to optimize, I'll drop them.
>>>>
>>>> There is a defect for blk-mq compare to blk-sq, specifically split io
>>>> will end up discontinuous if the device is under high io pressure, while
>>>> 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 woken 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 issuing 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=max_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 machine
>>>> 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 enabled,
>> the problem you mention above would be most likely completely invisible,
>> 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=/dev/$dev \
>>>> -name=test \
>>>> -ioengine=psync \
>>>> -allow_mounted_write=0 \
>>>> -group_reporting \
>>>> -direct=1 \
>>>> -offset_increment=1g \
>>>> -rw=randwrite \
>>>> -bs=1024k \
>>>> -numjobs={1,2,4,8,16,32,64,128,256,512} \
>>>> -runtime=110 \
>>>> -ramp_time=10
>>>>
>>>> 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 scheduler
>> 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 show
>> are the same for before-after patch with a number of threads <= 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 ?
>
> Hi,
>
> Sorry about the misleading of this test case.
>
> 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.
>
> 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.
>
> patch 6-8 will let split bios still be issued continuously under high
> pressure.
>
> Thanks,
> Kuai
>
--
Damien Le Moal
Western Digital Research
