On 1/7/20 8:55 AM, Mark Papadakis wrote: > This is perhaps an odd request, but if it’s trivial to implement > support for this described feature, it could help others like it ‘d > help me (I ‘ve been experimenting with io_uring for some time now). > > Being able to register an eventfd with an io_uring context is very > handy, if you e.g have some sort of reactor thread multiplexing I/O > using epoll etc, where you want to be notified when there are pending > CQEs to drain. The problem, such as it is, is that this can result in > un-necessary/spurious wake-ups. > > If, for example, you are monitoring some sockets for EPOLLIN, and when > poll says you have pending bytes to read from their sockets, and said > sockets are non-blocking, and for each some reported event you reserve > an SQE for preadv() to read that data and then you io_uring_enter to > submit the SQEs, because the data is readily available, as soon as > io_uring_enter returns, you will have your completions available - > which you can process. The “problem” is that poll will wake up > immediately thereafter in the next reactor loop iteration because > eventfd was tripped (which is reasonable but un-necessary). > > What if there was a flag for io_uring_setup() so that the eventfd > would only be tripped for CQEs that were processed asynchronously, or, > if that’s non-trivial, only for CQEs that reference file FDs? > > That’d help with that spurious wake-up. One easy way to do that would be for the application to signal that it doesn't want eventfd notifications for certain requests. Like using an IOSQE_ flag for that. Then you could set that on the requests you submit in response to triggering an eventfd event. -- Jens Axboe
