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[109.64.11.187]) by smtp.gmail.com with ESMTPSA id u24sm9049976qkm.40.2020.02.03.02.35.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Feb 2020 02:35:41 -0800 (PST) Date: Mon, 3 Feb 2020 05:35:36 -0500 From: "Michael S. Tsirkin" To: Sergio Lopez Subject: Re: [PATCH v2 2/4] virtio-scsi: default num_queues to -smp N Message-ID: <20200203053205-mutt-send-email-mst@kernel.org> References: <20200124100159.736209-1-stefanha@redhat.com> <20200124100159.736209-3-stefanha@redhat.com> <20200127141031.6e108839.cohuck@redhat.com> <20200129154438.GC157595@stefanha-x1.localdomain> <20200130105235.GC176651@stefanha-x1.localdomain> <20200203102529.3op54zggtquoguuo@dritchie> MIME-Version: 1.0 In-Reply-To: <20200203102529.3op54zggtquoguuo@dritchie> X-MC-Unique: lFHGe4R1MzWvBL2WLJtzuQ-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Fam Zheng , Eduardo Habkost , qemu-block@nongnu.org, Cornelia Huck , qemu-devel@nongnu.org, Max Reitz , Stefan Hajnoczi , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Feb 03, 2020 at 11:25:29AM +0100, Sergio Lopez wrote: > On Thu, Jan 30, 2020 at 10:52:35AM +0000, Stefan Hajnoczi wrote: > > On Thu, Jan 30, 2020 at 01:29:16AM +0100, Paolo Bonzini wrote: > > > On 29/01/20 16:44, Stefan Hajnoczi wrote: > > > > On Mon, Jan 27, 2020 at 02:10:31PM +0100, Cornelia Huck wrote: > > > >> On Fri, 24 Jan 2020 10:01:57 +0000 > > > >> Stefan Hajnoczi wrote: > > > >>> @@ -47,10 +48,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIP= roxy *vpci_dev, Error **errp) > > > >>> { > > > >>> VHostSCSIPCI *dev =3D VHOST_SCSI_PCI(vpci_dev); > > > >>> DeviceState *vdev =3D DEVICE(&dev->vdev); > > > >>> - VirtIOSCSICommon *vs =3D VIRTIO_SCSI_COMMON(vdev); > > > >>> + VirtIOSCSIConf *conf =3D &dev->vdev.parent_obj.parent_obj.co= nf; > > > >>> + > > > >>> + /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */ > > > >>> + if (conf->num_queues =3D=3D VIRTIO_SCSI_AUTO_NUM_QUEUES) { > > > >>> + conf->num_queues =3D current_machine->smp.cpus; > > > >> This now maps the request vqs 1:1 to the vcpus. What about the fix= ed > > > >> vqs? If they don't really matter, amend the comment to explain tha= t? > > > > The fixed vqs don't matter. They are typically not involved in the= data > > > > path, only the control path where performance doesn't matter. > > >=20 > > > Should we put a limit on the number of vCPUs? For anything above ~12= 8 > > > the guest is probably not going to be disk or network bound. > >=20 > > Michael Tsirkin pointed out there's a hard limit of VIRTIO_QUEUE_MAX > > (1024). We need to at least stay under that limit. > >=20 > > Should the guest have >128 virtqueues? Each virtqueue requires guest > > RAM and 2 host eventfds. Eventually these resource requirements will > > become a scalability problem, but how do we choose a hard limit and wha= t > > happens to guest performance above that limit? >=20 > From the UX perspective, I think it's safer to use a rather low upper > limit for the automatic configuration. >=20 > Users of large VMs (>=3D32 vCPUs) aiming for the optimal performance are > already facing the need of manually tuning (or relying on a software > to do that for them) other aspects of it, like vNUMA, IOThreads and > CPU pinning, so I don't think we should focus on this group. >=20 > On the other hand, the increase in host resource requirements may have > unforeseen in some environments, specially to virtio-blk users with > multiple disks. >=20 > All in all, I don't have data that would justify setting the limit to > one value or the other. The only argument I can put on the table is > that, so far, we only had one VQ per device, so perhaps a conservative > value (4? 8?) would make sense from a safety and compatibility point > of view. >=20 > Thanks, > Sergio. >=20 A bit more testing with different vcpu values can't hurt here ... Stefan? --=20 MST