Re: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load

[Damien Le Moal]

On 4/25/22 15:47, yukuai (C) wrote:
> 在 2022/04/25 14:23, Damien Le Moal 写道:
>> 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 ?
> 
> Hi,
> 
> It's right.
> 
> In fact, after this patchset, since each large io will need total 8
> tags, only 8 jobs can be in the queue while others are waiting for
> scheduler tag.
> 
>>
>> 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.
> 
> This solution sounds feasible in theory, however, I'm not sure yet how
> to implement that 'temporary excess'.

It should not be too hard.

By the way, did you check that doing something like:

echo 2048 > /sys/block/sdX/queue/nr_requests

improves performance for your high number of jobs test case ?

> 
> Thanks,
> Kuai
>>
>>
>>>
>>> patch 6-8 will let split bios still be issued continuously under high
>>> pressure.
>>>
>>> Thanks,
>>> Kuai
>>>
>>
>>


-- 
Damien Le Moal
Western Digital Research