On 2/25/2020 6:13 AM, Jens Axboe wrote: >>> I still think flags tagged on sqes could be a better choice, which >>> gives users an ability to deside if they want to ignore the cqes, not >>> only for links, but also for normal sqes. >>> >>> In addition, boxed cqes couldn’t resolve the issue of >>> IORING_IO_TIMEOUT. >> >> I would tend to agree, and it'd be trivial to just set the flag on >> whatever SQEs in the chain you don't care about. Or even an individual >> SQE, though that's probably a bit more of a reach in terms of use case. >> Maybe nop with drain + ignore? Flexible, but not performant. The existence of drain is already makes io_uring to do a lot of extra stuff, and even worse when it's actually used. >> >> In any case it's definitely more flexible. That's a different thing. Knowing how requests behave (e.g. if nbytes!=res, then fail link), one would want to get cqe for the last executed sqe, whether it's an error or a success for the last one. It makes a link to be handled as a single entity. I don't see a way to emulate similar behaviour with the unconditional masking. Probably, we will need them both. > In the interest of taking this to the extreme, I tried a nop benchmark > on my laptop (qemu/kvm). Granted, this setup is particularly sensitive > to spinlocks, they are a lot more expensive there than on a real host. > > Anyway, regular nops run at about 9.5M/sec with a single thread. > Flagging all SQEs with IOSQE_NO_CQE nets me about 14M/sec. So a handy > improvement. Looking at the top of profiles: > > cqe-per-sqe: > > + 28.45% io_uring [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore > + 14.38% io_uring [kernel.kallsyms] [k] io_submit_sqes > + 9.38% io_uring [kernel.kallsyms] [k] io_put_req > + 7.25% io_uring libc-2.31.so [.] syscall > + 6.12% io_uring [kernel.kallsyms] [k] kmem_cache_free > > no-cqes: > > + 19.72% io_uring [kernel.kallsyms] [k] io_put_req > + 11.93% io_uring [kernel.kallsyms] [k] io_submit_sqes > + 10.14% io_uring [kernel.kallsyms] [k] kmem_cache_free > + 9.55% io_uring libc-2.31.so [.] syscall > + 7.48% io_uring [kernel.kallsyms] [k] __io_queue_sqe > > I'll try the real disk IO tomorrow, using polled IO. Great, would love to see -- Pavel Begunkov
