Re: IORING_OP_POLL_ADD slower than linux-aio IOCB_CMD_POLL

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

On 6/15/22 12:04, Avi Kivity wrote:
> 
> On 15/06/2022 13.48, Pavel Begunkov wrote:
>> On 6/15/22 11:12, Avi Kivity wrote:
>>>
>>> On 19/04/2022 20.14, Jens Axboe wrote:
>>>> On 4/19/22 9:21 AM, Jens Axboe wrote:
>>>>> On 4/19/22 6:31 AM, Jens Axboe wrote:
>>>>>> On 4/19/22 6:21 AM, Avi Kivity wrote:
>>>>>>> On 19/04/2022 15.04, Jens Axboe wrote:
>>>>>>>> On 4/19/22 5:57 AM, Avi Kivity wrote:
>>>>>>>>> On 19/04/2022 14.38, Jens Axboe wrote:
>>>>>>>>>> On 4/19/22 5:07 AM, Avi Kivity wrote:
>>>>>>>>>>> A simple webserver shows about 5% loss compared to linux-aio.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I expect the loss is due to an optimization that io_uring lacks -
>>>>>>>>>>> inline completion vs workqueue completion:
>>>>>>>>>> I don't think that's it, io_uring never punts to a workqueue for
>>>>>>>>>> completions.
>>>>>>>>> I measured this:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>    Performance counter stats for 'system wide':
>>>>>>>>>
>>>>>>>>>            1,273,756 io_uring:io_uring_task_add
>>>>>>>>>
>>>>>>>>>         12.288597765 seconds time elapsed
>>>>>>>>>
>>>>>>>>> Which exactly matches with the number of requests sent. If that's the
>>>>>>>>> wrong counter to measure, I'm happy to try again with the correct
>>>>>>>>> counter.
>>>>>>>> io_uring_task_add() isn't a workqueue, it's task_work. So that is
>>>>>>>> expected.
>>>>> Might actually be implicated. Not because it's a async worker, but
>>>>> because I think we might be losing some affinity in this case. Looking
>>>>> at traces, we're definitely bouncing between the poll completion side
>>>>> and then execution the completion.
>>>>>
>>>>> Can you try this hack? It's against -git + for-5.19/io_uring. If you let
>>>>> me know what base you prefer, I can do a version against that. I see
>>>>> about a 3% win with io_uring with this, and was slower before against
>>>>> linux-aio as you saw as well.
>>>> Another thing to try - get rid of the IPI for TWA_SIGNAL, which I
>>>> believe may be the underlying cause of it.
>>>>
>>>
>>> Resurrecting an old thread. I have a question about timeliness of completions. Let's assume a request has completed. From the patch, it appears that io_uring will only guarantee that a completion appears on the completion ring if the thread has entered kernel mode since the completion happened. So user-space polling of the completion ring can cause unbounded delays.
>>
>> Right, but polling the CQ is a bad pattern, io_uring_{wait,peek}_cqe/etc.
>> will do the polling vs syscalling dance for you.
> 
> 
> Can you be more explicit?
> 
> 
> I don't think peek is enough. If there is a cqe pending, it will return it, but will not cause compeleted-but-unqueued events to generate completions.
> 
> 
> And wait won't enter the kernel if a cqe is pending, IIUC.

Right, usually it won't, but works if you eventually end up
waiting, e.g. by waiting for all expected cqes.


>> For larger audience, I'll remind that it's an opt-in feature
>>
> 
> I don't understand - what is an opt-in feature?

The behaviour that you worry about when CQEs are not posted until
you do syscall, it's only so if you set IORING_SETUP_COOP_TASKRUN.


>>> If this is correct (it's not unreasonable, but should be documented), then there should also be a simple way to force a kernel entry. But how to do this using liburing? IIUC if I the following apply:
>>>
>>>
>>>   1. I have no pending sqes
>>>
>>>   2. There are pending completions
>>>
>>>   3. There is a completed event for which a completion has not been appended to the completion queue ring
>>>
>>>
>>> Then io_uring_wait_cqe() will elide io_uring_enter() and the completed-but-not-reported event will be delayed.
>>
>> One way is to process all CQEs and then it'll try to enter the
>> kernel and do the job.
>>
>> Another way is to also set IORING_SETUP_TASKRUN_FLAG, then when
>> there is work that requires to enter the kernel io_uring will
>> set IORING_SQ_TASKRUN in sq_flags.
>> Actually, I'm not mistaken io_uring has some automagic handling
>> of it internally
>>
>> https://github.com/axboe/liburing/blob/master/src/queue.c#L36
>>
>>
>>

-- 
Pavel Begunkov