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. -- MST
