Re: [PATCH 4/5] io_uring: add support for batch wait timeout

[Jens Axboe <axboe@kernel.dk>]

On 8/20/24 3:10 PM, David Wei wrote:
> On 2024-08-19 16:28, Jens Axboe wrote:
>> Waiting for events with io_uring has two knobs that can be set:
>>
>> 1) The number of events to wake for
>> 2) The timeout associated with the event
>>
>> Waiting will abort when either of those conditions are met, as expected.
>>
>> This adds support for a third event, which is associated with the number
>> of events to wait for. Applications generally like to handle batches of
>> completions, and right now they'd set a number of events to wait for and
>> the timeout for that. If no events have been received but the timeout
>> triggers, control is returned to the application and it can wait again.
>> However, if the application doesn't have anything to do until events are
>> reaped, then it's possible to make this waiting more efficient.
>>
>> For example, the application may have a latency time of 50 usecs and
>> wanting to handle a batch of 8 requests at the time. If it uses 50 usecs
>> as the timeout, then it'll be doing 20K context switches per second even
>> if nothing is happening.
>>
>> This introduces the notion of min batch wait time. If the min batch wait
>> time expires, then we'll return to userspace if we have any events at all.
>> If none are available, the general wait time is applied. Any request
>> arriving after the min batch wait time will cause waiting to stop and
>> return control to the application.
> 
> I think the batch request count should be applied to the min_timeout,
> such that:
> 
> start_time          min_timeout            timeout
>     |--------------------|--------------------|
> 
> Return to user between [start_time, min_timeout) if there are wait_nr
> number of completions, checked by io_req_local_work_add(), or is it
> io_wake_function()?

Right, if we get the batch fulfilled, we should ALWAYS return.

If we have any events and min_timeout expires, return.

If not, sleep the full timeout.

> Return to user between [min_timeout, timeout) if there are at least one
> completion.

Yes

> Return to user at timeout always.

Yes

This should be how it works, and how I described it in the commit
message.

-- 
Jens Axboe