Re: IORING_OP_POLL_ADD slower than linux-aio IOCB_CMD_POLL

[Avi Kivity <avi@scylladb.com>]

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.
>

Won't it delay notification until the next io_uring_enter? Or does 
io_uring only guarantee completions when you call it (and earlier 
completions are best-effort?)


For Seastar it's not a problem, asking about the general io_uring 
completion philosophy.


I'll try it tomorrow (also the other patch).