[PATCH] virtio_ring: use direct descriptor as far as possible

[Ming Lei]

Direct descriptor can avoid one kmalloc() and should have
better cache utilization, so try to use it if there are enough
free direct descriptors.

Suggested-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4d08f45a..c76b7a4 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -224,9 +224,13 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
 	total_sg = total_in + total_out;
 
-	/* If the host supports indirect descriptor tables, and we have multiple
-	 * buffers, then go indirect. FIXME: tune this threshold */
-	if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
+	/*
+	 * If the host supports indirect descriptor tables, and we have
+	 * multiple buffers, then go indirect only if free descriptors
+	 * are less than 16.
+	 */
+	if (vq->indirect && total_sg > 1 && vq->vq.num_free &&
+			vq->vq.num_free < 16) {
 		head = vring_add_indirect(vq, sgs, next, total_sg, total_out,
 					  total_in,
 					  out_sgs, in_sgs, gfp);
-- 
1.7.9.5

 

> 
> And we might consider flipping the default, as most would probably like
> to have the sg merging on.
>

With this patch, I think SG merge can be kept as off at default if there is
no extra cost introduced for handling more segments by driver or HW.

If SG merge is changed to on in my above virtio-blk fio test, small throughput
drop can be observed. 


Thanks,
-- 
Ming Lei
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