Messages in this thread | | | Subject | Re: [PATCH -next RFC v3 0/8] improve tag allocation under heavy load | From | "yukuai (C)" <> | Date | Sun, 24 Apr 2022 10:43:22 +0800 |
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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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