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

[yukuai (C)]

在 2022/04/25 14:50, Damien Le Moal 写道:
> 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.

I'll try to figure out a proper way, in the meantime, any suggestions
would be appreciated.
> 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 ?

Yes, performance will not degrade when numjobs is not greater than 256
in this case.
> > Thanks,
> > Kuai
> > > > patch 6-8 will let split bios still be issued continuously under high
> > > > pressure.
> > > >
> > > > Thanks,
> > > > Kuai