Currently if VIRTIO_RING_F_INDIRECT_DESC is enabled we will use indirect
descriptors even if we have plenty of space in the ring. This means that
we take a performance hit at all times due to the overhead of creating
indirect descriptors.
With this patch, we will use indirect descriptors only if we have less than
either 16, or 12% of the total amount of descriptors available.
I did basic performance benchmark on virtio-net with vhost enabled.
Before:
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec
87380 16384 16384 10.00 4563.92
After:
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec
87380 16384 16384 10.00 5353.28
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <levinsasha928@xxxxxxxxx>
---
drivers/virtio/virtio_ring.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c7a2c20..5e0ce15 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -82,6 +82,7 @@ struct vring_virtqueue
/* Host supports indirect buffers */
bool indirect;
+ unsigned int indirect_threshold;
/* Host publishes avail event idx */
bool event;
@@ -176,8 +177,9 @@ int virtqueue_add_buf_gfp(struct virtqueue *_vq,
BUG_ON(data == NULL);
/* If the host supports indirect descriptor tables, and we have multiple
- * buffers, then go indirect. FIXME: tune this threshold */
- if (vq->indirect && (out + in) > 1 && vq->num_free) {
+ * buffers, then go indirect. */
+ if (vq->indirect && (out + in) > 1 &&
+ (vq->num_free < vq->indirect_threshold)) {
head = vring_add_indirect(vq, sg, out, in, gfp);
if (likely(head >= 0))
goto add_head;
@@ -485,6 +487,12 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
#endif
vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
+ /*
+ * Use indirect descriptors only when we have less than either 12%
+ * or 16 of the descriptors in the ring available.
+ */
+ if (vq->indirect)
+ vq->indirect_threshold = max(16U, num >> 3);
vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
/* No callback? Tell other side not to bother us. */
--
1.7.8.rc3
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