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 <[email protected]>
> Signed-off-by: Stefan Hajnoczi <[email protected]>
> ---
> 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
