Re: [PATCH v2 0/2] Optimise io_uring completion waiting

[Pavel Begunkov]

On 24/09/2019 11:27, Jens Axboe wrote:
> On 9/24/19 2:02 AM, Jens Axboe wrote:
>> On 9/24/19 1:06 AM, Pavel Begunkov wrote:
>>> On 24/09/2019 02:00, Jens Axboe wrote:
>>>>> I think we can do the same thing, just wrapping the waitqueue in a
>>>>> structure with a count in it, on the stack. Got some flight time
>>>>> coming up later today, let me try and cook up a patch.
>>>>
>>>> Totally untested, and sent out 5 min before departure... But something
>>>> like this.
>>> Hmm, reminds me my first version. Basically that's the same thing but
>>> with macroses inlined. I wanted to make it reusable and self-contained,
>>> though.
>>>
>>> If you don't think it could be useful in other places, sure, we could do
>>> something like that. Is that so?
>>
>> I totally agree it could be useful in other places. Maybe formalized and
>> used with wake_up_nr() instead of adding a new primitive? Haven't looked
>> into that, I may be talking nonsense.
>>
>> In any case, I did get a chance to test it and it works for me. Here's
>> the "finished" version, slightly cleaned up and with a comment added
>> for good measure.
> 
> Notes:
> 
> This version gets the ordering right, you need exclusive waits to get
> fifo ordering on the waitqueue.
> 
> Both versions (yours and mine) suffer from the problem of potentially
> waking too many. I don't think this is a real issue, as generally we
> don't do threaded access to the io_urings. But if you had the following
> tasks wait on the cqring:
> 
> [min_events = 32], [min_events = 8], [min_events = 8]
> 
> and we reach the io_cqring_events() == threshold, we'll wake all three.
> I don't see a good solution to this, so I suspect we just live with
> until proven an issue. Both versions are much better than what we have
> now.
> 
If io_cqring_events() == 8, only the last two would be woken up in both
implementations, as to_wait/threshold is specified per waiter. Isn't it?

Agree with waiting, I don't see a good real-life case for that, that
couldn't be done efficiently in userspace.


-- 
Yours sincerely,
Pavel Begunkov

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