Re: [PATCH for-5.16 v3 0/8] task work optimization

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/28/21 12:46, Hao Xu wrote:
> 在 2021/10/28 下午2:07, Hao Xu 写道:
>> 在 2021/10/28 上午2:15, Pavel Begunkov 写道:
>>> On 10/27/21 15:02, Hao Xu wrote:
>>>> Tested this patchset by manually replace __io_queue_sqe() in
>>>> io_queue_sqe() by io_req_task_queue() to construct 'heavy' task works.
>>>> Then test with fio:
>>>
>>> If submissions and completions are done by the same task it doesn't
>>> really matter in which order they're executed because the task won't
>>> get back to userspace execution to see CQEs until tw returns.
>> It may matter, it depends on the time cost of submittion
>> and the DMA IO time. Pick up sqpoll mode as example,
>> we submit 10 reqs:
>> t1          io_submit_sqes
>>              -->io_req_task_queue
>> t2          io_task_work_run
>> we actually do the submittion in t2,  but if the workload
>> is big engough, the 'irq completion TW' will be inserted
>> to the TW list after t2 is fully done, then those
>> 'irq completion TW' will be delayed to the next round.
>> With this patchset, we can handle them first.
>>> Furthermore, it even might be worse because the earlier you submit
>>> the better with everything else equal.
>>>
>>> IIRC, that's how it's with fio, right? If so, you may get better
>>> numbers with a test that does submissions and completions in
>>> different threads.
>> Because of the completion cache, I doubt if it works.
>> For single ctx, it seems we always update the cqring
>> pointer after all the TWs in the list are done.
> I suddenly realized sqpoll mode does submissions and completions
> in different threads, and in this situation this patchset always
> first commit_cqring() after handling TWs in priority list.
> So this is the right test, do I miss something?

Yep, should be it. So the scope of the feature is SQPOLL or
completion/submission with different tasks.

>>>
>>> Also interesting to find an explanation for you numbers assuming
>> The reason may be what I said above, but I don't have a
>> strict proof now.
>>> they're stable. 7/8 batching? How often it does it go this path?
>>> If only one task submits requests it should already be covered
>>> with existing batching.
>> the problem of the existing batch is(given there is only
>> one ctx):
>> 1. we flush it after all the TWs done
>> 2. we batch them if we have uring lock.
>> the new batch is:
>> 1. don't care about uring lock
>> 2. we can flush the completions in the priority list
>>     in advance.(which means userland can see it earlier.)
>>>
>>>
>>>> ioengine=io_uring
>>>> sqpoll=1
>>>> thread=1
>>>> bs=4k
>>>> direct=1
>>>> rw=randread
>>>> time_based=1
>>>> runtime=600
>>>> randrepeat=0
>>>> group_reporting=1
>>>> filename=/dev/nvme0n1
>>>>
>>>> 2/8 set unlimited priority_task_list, 8/8 set a limitation of
>>>> 1/3 * (len_prority_list + len_normal_list), data below:
>>>>     depth     no 8/8   include 8/8      before this patchset
>>>>      1        7.05         7.82              7.10
>>>>      2        8.47         8.48              8.60
>>>>      4        10.42        9.99              10.42
>>>>      8        13.78        13.13             13.22
>>>>      16       27.41        27.92             24.33
>>>>      32       49.40        46.16             53.08
>>>>      64       102.53       105.68            103.36
>>>>      128      196.98       202.76            205.61
>>>>      256      372.99       375.61            414.88
>>>>      512      747.23       763.95            791.30
>>>>      1024     1472.59      1527.46           1538.72
>>>>      2048     3153.49      3129.22           3329.01
>>>>      4096     6387.86      5899.74           6682.54
>>>>      8192     12150.25     12433.59          12774.14
>>>>      16384    23085.58     24342.84          26044.71
>>>>
>>>> It seems 2/8 is better, haven't tried other choices other than 1/3,
>>>> still put 8/8 here for people's further thoughts.
>>>>
>>>> Hao Xu (8):
>>>>    io-wq: add helper to merge two wq_lists
>>>>    io_uring: add a priority tw list for irq completion work
>>>>    io_uring: add helper for task work execution code
>>>>    io_uring: split io_req_complete_post() and add a helper
>>>>    io_uring: move up io_put_kbuf() and io_put_rw_kbuf()
>>>>    io_uring: add nr_ctx to record the number of ctx in a task
>>>>    io_uring: batch completion in prior_task_list
>>>>    io_uring: add limited number of TWs to priority task list
>>>>
>>>>   fs/io-wq.h    |  21 +++++++
>>>>   fs/io_uring.c | 168 +++++++++++++++++++++++++++++++++++---------------
>>>>   2 files changed, 138 insertions(+), 51 deletions(-)
>>>>
>>>
> 

-- 
Pavel Begunkov