Re: [PATCH v4 12/12] block/io_uring: use non-vectored read/write when possible

[Kevin Wolf <kwolf@redhat.com>]

Am 10.09.2025 um 19:57 hat Stefan Hajnoczi geschrieben:
> The io_uring_prep_readv2/writev2() man pages recommend using the
> non-vectored read/write operations when possible for performance
> reasons.
> 
> I didn't measure a significant difference but it doesn't hurt to have
> this optimization in place.
> 
> Suggested-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/io_uring.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/block/io_uring.c b/block/io_uring.c
> index dd930ee57e..bbefbddcc0 100644
> --- a/block/io_uring.c
> +++ b/block/io_uring.c
> @@ -49,12 +49,24 @@ static void luring_prep_sqe(struct io_uring_sqe *sqe, void *opaque)
>  #ifdef HAVE_IO_URING_PREP_WRITEV2
>      {
>          int luring_flags = (flags & BDRV_REQ_FUA) ? RWF_DSYNC : 0;
> -        io_uring_prep_writev2(sqe, fd, qiov->iov,
> -                              qiov->niov, offset, luring_flags);
> +        if (luring_flags != 0 || qiov->niov > 1) {
> +            io_uring_prep_writev2(sqe, fd, qiov->iov,
> +                                  qiov->niov, offset, luring_flags);
> +        } else {
> +            /* The man page says non-vectored is faster than vectored */
> +            struct iovec *iov = qiov->iov;
> +            io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, offset);
> +        }
>      }
>  #else
>          assert(flags == 0);
> -        io_uring_prep_writev(sqe, fd, qiov->iov, qiov->niov, offset);
> +        if (qiov->niov > 1) {
> +            io_uring_prep_writev(sqe, fd, qiov->iov, qiov->niov, offset);
> +        } else {
> +            /* The man page says non-vectored is faster than vectored */
> +            struct iovec *iov = qiov->iov;
> +            io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, offset);
> +        }
>  #endif

We have a lot of duplication in this now. Let's use the #ifdef a little
more locally:

    {
        int luring_flags = (flags & BDRV_REQ_FUA) ? RWF_DSYNC : 0;
        if (luring_flags != 0 || qiov->niov > 1) {
#ifdef HAVE_IO_URING_PREP_WRITEV2
            io_uring_prep_writev2(sqe, fd, qiov->iov,
                                  qiov->niov, offset, luring_flags);
#else
            assert(luring_flags == 0);
            io_uring_prep_writev(sqe, fd, qiov->iov, qiov->niov, offset);
#endif
        } else {
            /* The man page says non-vectored is faster than vectored */
            struct iovec *iov = qiov->iov;
            io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, offset);
        }
    }

Kevin