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SubjectRe: [PATCH net-next v2 3/5] virtio_ring: add packed ring support
On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
>
> On 2018/11/8 上午9:38, Tiwei Bie wrote:
> > > > +
> > > > + if (vq->vq.num_free < descs_used) {
> > > > + pr_debug("Can't add buf len %i - avail = %i\n",
> > > > + descs_used, vq->vq.num_free);
> > > > + /* FIXME: for historical reasons, we force a notify here if
> > > > + * there are outgoing parts to the buffer. Presumably the
> > > > + * host should service the ring ASAP. */
> > > I don't think we have a reason to do this for packed ring.
> > > No historical baggage there, right?
> > Based on the original commit log, it seems that the notify here
> > is just an "optimization". But I don't quite understand what does
> > the "the heuristics which KVM uses" refer to. If it's safe to drop
> > this in packed ring, I'd like to do it.
>
>
> According to the commit log, it seems like a workaround of lguest networking
> backend. I agree to drop it, we should not have such burden.
>
> But we should notice that, with this removed, the compare between packed vs
> split is kind of unfair.

I don't think this ever triggers to be frank. When would it?

> Consider the removal of lguest support recently,
> maybe we can drop this for split ring as well?
>
> Thanks

If it's helpful, then for sure we can drop it for virtio 1.
Can you see any perf differences at all? With which device?

>
> >
> > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > Author: Rusty Russell<rusty@rustcorp.com.au>
> > Date: Fri Jul 25 12:06:04 2008 -0500
> >
> > virtio: don't always force a notification when ring is full
> > We force notification when the ring is full, even if the host has
> > indicated it doesn't want to know. This seemed like a good idea at
> > the time: if we fill the transmit ring, we should tell the host
> > immediately.
> > Unfortunately this logic also applies to the receiving ring, which is
> > refilled constantly. We should introduce real notification thesholds
> > to replace this logic. Meanwhile, removing the logic altogether breaks
> > the heuristics which KVM uses, so we use a hack: only notify if there are
> > outgoing parts of the new buffer.
> > Here are the number of exits with lguest's crappy network implementation:
> > Before:
> > network xmit 7859051 recv 236420
> > After:
> > network xmit 7858610 recv 118136
> > Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 72bf8bc09014..21d9a62767af 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
> > if (vq->num_free < out + in) {
> > pr_debug("Can't add buf len %i - avail = %i\n",
> > out + in, vq->num_free);
> > - /* We notify*even if* VRING_USED_F_NO_NOTIFY is set here. */
> > - vq->notify(&vq->vq);
> > + /* FIXME: for historical reasons, we force a notify here if
> > + * there are outgoing parts to the buffer. Presumably the
> > + * host should service the ring ASAP. */
> > + if (out)
> > + vq->notify(&vq->vq);
> > END_USE(vq);
> > return -ENOSPC;
> > }
> >
> >

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