On Thu, Dec 20, 2018 at 05:47:55PM +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.
>
> Also, move the write barrier in logging cache sync, so that it
> is done only when logging is enabled. It means there is now
> one more barrier for split ring when logging is enabled.
>
> With Kernel's pktgen benchmark, ~3% performance gain is measured.
>
> Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
Acked-by: Michael S. Tsirkin <m...@redhat.com>
> ---
> lib/librte_vhost/vhost.h | 2 ++
> lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---
> 2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index 3b3265c4b..7d1d8a308 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -457,6 +457,8 @@ vhost_log_cache_sync(struct virtio_net *dev, struct
> vhost_virtqueue *vq)
> !dev->log_base))
> return;
>
> + rte_smp_wmb();
> +
> log_base = (unsigned long *)(uintptr_t)dev->log_base;
>
> /*
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index 8c657a101..02c1fd3a4 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,
> + head_idx *
> + sizeof(struct vring_packed_desc),
> + sizeof(struct vring_packed_desc));
> +
> vq->shadow_used_idx = 0;
> vhost_log_cache_sync(dev, vq);
> }
> --
> 2.17.2
