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Can we increase this limit? To: Stefan Hajnoczi References: <20200805121107.GG361702@stefanha-x1.localdomain> <20200805081144-mutt-send-email-mst@kernel.org> <20200806123708.GC379937@stefanha-x1.localdomain> <20200807095953.GA600298@stefanha-x1.localdomain> From: Jason Wang Message-ID: Date: Mon, 10 Aug 2020 17:49:27 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200807095953.GA600298@stefanha-x1.localdomain> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=205.139.110.120; envelope-from=jasowang@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/10 05:06:51 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: Yajun Wu , "qemu-devel@nongnu.org" , "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020/8/7 下午5:59, Stefan Hajnoczi wrote: > On Fri, Aug 07, 2020 at 11:35:08AM +0800, Jason Wang wrote: >> On 2020/8/6 下午8:37, Stefan Hajnoczi wrote: >>> On Wed, Aug 05, 2020 at 08:13:29AM -0400, Michael S. Tsirkin wrote: >>>> On Wed, Aug 05, 2020 at 01:11:07PM +0100, Stefan Hajnoczi wrote: >>>>> On Thu, Jul 30, 2020 at 07:46:09AM +0000, Yajun Wu wrote: >>>>>> I'm doing iperf test on VIRTIO net through vhost-user(HW VDPA). >>>>>> Find maximal acceptable tx_queue_size/rx_queue_size is 1024. >>>>>> Basically increase queue size can get better RX rate for my case. >>>>>> >>>>>> Can we increase the limit(VIRTQUEUE_MAX_SIZE) to 8192 to possibly gain better performance? >>>>> Hi, >>>>> The VIRTIO 1.1 specification says the maximum number of descriptors is >>>>> 32768 for both split and packed virtqueues. >>>>> >>>>> The vhost kernel code seems to support 32768. >>>>> >>>>> The 1024 limit is an implementation limit in QEMU. Increasing it would >>>>> require QEMU code changes. For example, VIRTQUEUE_MAX_SIZE is used as >>>>> the size of arrays. >>>>> >>>>> I can't think of a fundamental reason why QEMU needs to limit itself to >>>>> 1024 descriptors. Raising the limit would require fixing up the code and >>>>> ensuring that live migration remains compatible with older versions of >>>>> QEMU. >>>>> >>>>> Stefan >>>> There's actually a reason for a limit: in theory the vq size >>>> also sets a limit on the number of scatter/gather entries. >>>> both QEMU and vhost can't handle a packet split over > 1k chunks. >>>> >>>> We could add an extra limit for s/g size like block and scsi do, >>>> this will need spec, guest and host side work. >>> Interesting, thanks for explaining! This could be made explicit by >>> changing the QEMU code to: >>> >>> include/hw/virtio/virtio.h:#define VIRTQUEUE_MAX_SIZE IOV_MAX >>> >>> Looking more closely at the vhost kernel code I see that UIO_MAXIOV is >>> used in some places but not in vhost_vring_set_num() (ioctl >>> VHOST_SET_VRING_NUM). Is there a reason why UIO_MAXIOV isn't enforced >>> when the application sets the queue size? >> >> Actually three things: >> >> 1) queue size >> 2) #descriptors in a list >> 3) IOV size >> >> Spec limit the 2) to 1) but 2) may not equal to 3). >> >> So enforcing UIO_MAXIOV can not solve the problem completely. >> >> For vhost-net, it depends on socket to build skb which requires an iov array >> to work. We need remove this limitation by: >> >> - build skb by vhost-net itself >> - do piecewise copying >> >> Then we're not limited with #iov and more and support up to what spec >> supports. > If I understand correctly, you are saying vhost_net.ko does not support > more than UIO_MAXIOV descriptors today but it could be fixed as you > described. Yes, but it needs major refactoring on vhost_net. thanks > > Thanks! > > Stefan