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

[Hao Xu <haoxu@linux.alibaba.com>]

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?
>>
>> 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(-)
>>>
>>