Re: [RFC v2 2/3] io_uring: add fixed poll support

[Jens Axboe <axboe@kernel.dk>]

On 10/28/21 6:28?AM, Xiaoguang Wang wrote:
> Recently I spend time to research io_uring's fast-poll and multi-shot's
> performance using network echo-server model. Previously I always thought
> fast-poll is better than multi-shot and will give better performance,
> but indeed multi-shot is almost always better than fast-poll in real
> test, which is very interesting. I use ebpf to have some measurements,
> it shows that whether fast-poll is excellent or not depends entirely on
> that the first nowait try in io_issue_sqe() succeeds or fails. Take
> io_recv operation as example(recv buffer is 16 bytes):
>   1) the first nowait succeeds, a simple io_recv() is enough.
> In my test machine, successful io_recv() consumes 1110ns averagely.
> 
>   2) the first nowait fails, then we'll have some expensive work, which
> contains failed io_revc(), apoll allocations, vfs_poll(), miscellaneous
> initializations anc check in __io_arm_poll_handler() and a final
> successful io_recv(). Among then:
>     failed io_revc() consumes 620ns averagely.
>     vfs_poll() consumes 550ns averagely.
> I don't measure other overhead yet, but we can see if the first nowait
> try fails, we'll need at least 2290ns(620 + 550 + 1110) to complete it.
> In my echo server tests, 40% of first nowait io_recv() operations fails.
> 
> From above measurements, it can explain why mulit-shot is better than
> multi-shot, mulit-shot can ensure the first nowait try succeed.
> 
> Based on above measurements, I try to improve fast-poll a bit:
> Introduce fix poll support, currently it only works in file registered
> mode. With this feature, we can get rid of various repeated operations
> in io_arm_poll_handler(), contains apoll allocations, and miscellaneous
> initializations anc check.

I was toying with an idea on how to do persistent poll support,
basically moving the wait_queue_entry out of io_poll and hence detaching
it from the io_kiocb. That would allow a per-file (and type) poll entry
to remain persistent in the kernel rather than needing to do this
expensive work repeatedly. Pavel kindly reminded me of your work, which
unfortunately I had totally forgotten.

Did you end up taking this further? My idea was to make it work
independently of fixed files, but I also don't want to reinvent the
wheel if you ended up with something like this.

-- 
Jens Axboe