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Wed, 15 Oct 2025 14:44:34 +0000 (UTC) Received: from localhost (unknown [10.2.16.91]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 48BD91800446; Wed, 15 Oct 2025 14:44:33 +0000 (UTC) Date: Tue, 14 Oct 2025 15:52:43 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-devel@nongnu.org, Aarushi Mehta , Fam Zheng , Stefano Garzarella , Hanna Czenczek , eblake@redhat.com, qemu-block@nongnu.org, hibriansong@gmail.com, Stefan Weil , Paolo Bonzini Subject: Re: [PATCH v4 12/12] block/io_uring: use non-vectored read/write when possible Message-ID: <20251014195243.GH18850@fedora> References: <20250910175703.374499-1-stefanha@redhat.com> <20250910175703.374499-13-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="HwsL3XmgrE9900qX" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -10 X-Spam_score: -1.1 X-Spam_bar: - X-Spam_report: (-1.1 / 5.0 requ) BAYES_00=-1.9, DATE_IN_PAST_12_24=1.049, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --HwsL3XmgrE9900qX Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 10, 2025 at 06:33:40PM +0200, Kevin Wolf wrote: > 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. > >=20 > > I didn't measure a significant difference but it doesn't hurt to have > > this optimization in place. > >=20 > > Suggested-by: Eric Blake > > Signed-off-by: Stefan Hajnoczi > > --- > > block/io_uring.c | 29 ++++++++++++++++++++++++----- > > 1 file changed, 24 insertions(+), 5 deletions(-) > >=20 > > 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 *sq= e, void *opaque) > > #ifdef HAVE_IO_URING_PREP_WRITEV2 > > { > > int luring_flags =3D (flags & BDRV_REQ_FUA) ? RWF_DSYNC : 0; > > - io_uring_prep_writev2(sqe, fd, qiov->iov, > > - qiov->niov, offset, luring_flags); > > + if (luring_flags !=3D 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 =3D qiov->iov; > > + io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, = offset); > > + } > > } > > #else > > assert(flags =3D=3D 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, offse= t); > > + } else { > > + /* The man page says non-vectored is faster than vectored = */ > > + struct iovec *iov =3D qiov->iov; > > + io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, = offset); > > + } > > #endif >=20 > We have a lot of duplication in this now. Let's use the #ifdef a little > more locally: >=20 > { > int luring_flags =3D (flags & BDRV_REQ_FUA) ? RWF_DSYNC : 0; > if (luring_flags !=3D 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 =3D=3D 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 =3D qiov->iov; > io_uring_prep_write(sqe, fd, iov->iov_base, iov->iov_len, off= set); > } > } Will fix in v5. Thanks! 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