Message-ID: <55e8b04f-0d2f-2ce1-6514-5abd0b67fd48@opensource.wdc.com>
Date:   Mon, 25 Apr 2022 15:23:06 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     "yukuai (C)" <yukuai3@...wei.com>, axboe@...nel.dk,
        bvanassche@....org, andriy.shevchenko@...ux.intel.com,
        john.garry@...wei.com, ming.lei@...hat.com, qiulaibin@...wei.com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load

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

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