Re: Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit?

[Jason Wang]

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