Re: [dpdk-dev] [PATCH v2] vhost: batch used descs chains write-back with packed ring

[Maxime Coquelin]

On 12/20/18 5:44 AM, 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.

Oh yes, thanks for spotting this.


+                               sizeof(struct vring_packed_desc),
+                               sizeof(struct vring_packed_desc));
+
        vq->shadow_used_idx = 0;

A wmb() is needed before log_cache_sync?

I think you're right, I was wrong but thought we had a barrier in cache
sync function.
That's not very important for x86, but I think it should be preferable 
to do it in vhost_log_cache_sync(), if logging is enabled.

What do you think?

        vhost_log_cache_sync(dev, vq);
  }
--
2.17.2