Re: [RFC][PATCH 0/3] allow to skip CQE posting

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/11/21 2:51 PM, Pavel Begunkov wrote:
> It's expensive enough to post an CQE, and there are other
> reasons to want to ignore them, e.g. for link handling and
> it may just be more convenient for the userspace.

FWIW, tried with some benchmark doing QD1 buffered reads, reads
taking 60+% (memcpy/etc.) of the time and 20-25% for io_uring.
Got +6-7% to performance from the kernel side only with additional
room to make the userspace faster.

> Try to cover most of the use cases with one flag. The overhead
> is one "if (cqe->flags & IOSQE_CQE_SKIP_SUCCESS)" check per
> requests and a bit bloated req_set_fail(), should be bearable.
> 
> See 2/3 for the actual description of the flag.
> 
> Pavel Begunkov (3):
>   io_uring: clean cqe filling functions
>   io_uring: add option to skip CQE posting
>   io_uring: don't spinlock when not posting CQEs
> 
>  fs/io_uring.c                 | 103 ++++++++++++++++++++++------------
>  include/uapi/linux/io_uring.h |   3 +
>  2 files changed, 70 insertions(+), 36 deletions(-)
> 

-- 
Pavel Begunkov