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? Stefan
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