On Thu, Dec 20, 2018 at 12:44:46PM +0800, Tiwei Bie wrote:
> On Wed, Dec 19, 2018 at 10:29:52AM +0100, Maxime Coquelin wrote:
> > Instead of writing back descriptors chains in order, let's
> > write the first chain flags last in order to improve batching.
> >
> > With Kernel's pktgen benchmark, ~3% performance gain is measured.
> >
> > Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
> > ---
> >
> > V2:
> > Revert back to initial implementation to have a write
> > barrier before every descs flags store, but still
> > store first desc flags last. (Missing barrier reported
> > by Ilya)
> >
> >
> > lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
> > 1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> > index 8c657a101..de436af79 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -97,6 +97,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > {
> > int i;
> > uint16_t used_idx = vq->last_used_idx;
> > + uint16_t head_idx = vq->last_used_idx;
> > + uint16_t head_flags = 0;
> >
> > /* Split loop in two to save memory barriers */
> > for (i = 0; i < vq->shadow_used_idx; i++) {
> > @@ -126,12 +128,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > flags &= ~VRING_DESC_F_AVAIL;
> > }
> >
> > - vq->desc_packed[vq->last_used_idx].flags = flags;
> > + if (i > 0) {
> > + vq->desc_packed[vq->last_used_idx].flags = flags;
> >
> > - vhost_log_cache_used_vring(dev, vq,
> > + vhost_log_cache_used_vring(dev, vq,
> > vq->last_used_idx *
> > sizeof(struct vring_packed_desc),
> > sizeof(struct vring_packed_desc));
> > + } else {
> > + head_idx = vq->last_used_idx;
> > + head_flags = flags;
> > + }
> >
> > vq->last_used_idx += vq->shadow_used_packed[i].count;
> > if (vq->last_used_idx >= vq->size) {
> > @@ -140,7 +147,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
> > }
> > }
> >
> > - rte_smp_wmb();
> > + vq->desc_packed[head_idx].flags = head_flags;
> > +
> > + vhost_log_cache_used_vring(dev, vq,
> > + vq->last_used_idx *
>
> Should be head_idx.
>
> > + sizeof(struct vring_packed_desc),
> > + sizeof(struct vring_packed_desc));
> > +
> > vq->shadow_used_idx = 0;
>
> A wmb() is needed before log_cache_sync?
>
> > vhost_log_cache_sync(dev, vq);
Probably smarter to have the wmb in there.
This way we can skip it if not logging.
> > }
> > --
> > 2.17.2
> >
