support suspend/resume SQPOLL on inflight io-uring instance

support suspend/resume SQPOLL on inflight io-uring instance

[陈云星]

io-uring instance can be set flag : IO_URING_SETUP_SQPOLL to enable
kernel thread polling SQ. but some application have random io
behavior, for example:
     a). a burst read/write for a while,
     b). then many scatter small io operations.

during a) period, use sqpoll will be beneficial to performance.
but during b) period, use sqpoll will waste cpu time for empty busy poll.

If we can have a method to tell kernel start/stop poll sq when we
need, it is good for both of a) and b):
     in a) period, io_uring_enter tell kernel. start to poll
     after a) , io_uring_enter tell kernel stop polling and.

Re: support suspend/resume SQPOLL on inflight io-uring instance

[Jens Axboe]

On 6/4/20 9:38 PM, 陈云星 wrote:
> io-uring instance can be set flag : IO_URING_SETUP_SQPOLL to enable
> kernel thread polling SQ. but some application have random io
> behavior, for example:
>      a). a burst read/write for a while,
>      b). then many scatter small io operations.
> 
> during a) period, use sqpoll will be beneficial to performance.
> but during b) period, use sqpoll will waste cpu time for empty busy poll.
> 
> If we can have a method to tell kernel start/stop poll sq when we
> need, it is good for both of a) and b):
>      in a) period, io_uring_enter tell kernel. start to poll
>      after a) , io_uring_enter tell kernel stop polling and.

We already have the start side of it, that's io_uring_enter() which should
be done when IORING_SQ_NEED_WAKEUP is set. I do like your suggestion of
making it go to sleep on demand instead of always waiting for the grace
period.

We recently added IORING_CQ_EVENTFD_DISABLED, and we can use the same
area for this as well. Eg add IORING_CQ_SQPOLL_SLEEP or something like
that, and have the sq thread look for it. If set, it'll stop busy looping
and go to sleep.

-- 
Jens Axboe