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SubjectRe: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load
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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 |
>
> 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 |
>
> Yu Kuai (8):
> sbitmap: record the number of waiters for each waitqueue
> blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()
> sbitmap: make sure waitqueues are balanced
> blk-mq: don't preempt tag under heavy load
> sbitmap: force tag preemption if free tags are sufficient
> blk-mq: force tag preemption for split bios
> blk-mq: record how many tags are needed for splited bio
> sbitmap: wake up the number of threads based on required tags
>
> block/blk-merge.c | 8 +-
> block/blk-mq-tag.c | 49 +++++++++----
> block/blk-mq.c | 54 +++++++++++++-
> block/blk-mq.h | 4 +
> include/linux/blk_types.h | 4 +
> include/linux/sbitmap.h | 9 +++
> lib/sbitmap.c | 149 +++++++++++++++++++++++++++-----------
> 7 files changed, 216 insertions(+), 61 deletions(-)
>

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