On Tue, 12 May 2009 02:40:38 am Mark McLoughlin wrote:
> Still have one FIXME in the patch worth looking at - at what point
> should we use an indirect entry rather than consuming N entries?
Is this overkill?
Rusty.
virtio: use indirect buffers based on demand (heuristic)
virtio_ring uses a ring buffer of descriptors: indirect support allows
a single descriptor to refer to a table of descriptors. This saves
space in the ring, but requires a kmalloc/kfree.
Rather than try to figure out what the right threshold at which to use
indirect buffers, we drop the threshold dynamically when the ring is
under stress.
Note: to stress this, I reduced the ring size to 32 in lguest, and a
1G send reduced the threshold to 9.
Note2: I moved the BUG_ON()s above the indirect test, where they belong
(indirect falls thru on OOM, so the constraints still apply).
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -63,6 +63,8 @@ struct vring_virtqueue
/* Host supports indirect buffers */
bool indirect;
+ /* Threshold before we go indirect. */
+ unsigned int indirect_threshold;
/* Number of free buffers */
unsigned int num_free;
@@ -137,6 +141,32 @@ static int vring_add_indirect(struct vri
return head;
}
+static void adjust_threshold(struct vring_virtqueue *vq,
+ unsigned int out, unsigned int in)
+{
+ /* There are really two species of virtqueue, and it matters here.
+ * If there are no output parts, it's a "normally full" receive queue,
+ * otherwise it's a "normally empty" send queue. */
+ if (out) {
+ /* Leave threshold unless we're full. */
+ if (out + in < vq->num_free)
+ return;
+ } else {
+ /* Leave threshold unless we're empty. */
+ if (vq->num_free != vq->vring.num)
+ return;
+ }
+
+ /* Never drop threshold below 1 */
+ vq->indirect_threshold /= 2;
+ vq->indirect_threshold |= 1;
+
+ printk("%s %s: indirect threshold %u (%u+%u vs %u)\n",
+ dev_name(&vq->vq.vdev->dev),
+ vq->vq.name, vq->indirect_threshold,
+ out, in, vq->num_free);
+}
+
static int vring_add_buf(struct virtqueue *_vq,
struct scatterlist sg[],
unsigned int out,
@@ -149,18 +179,31 @@ static int vring_add_buf(struct virtqueu
START_USE(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) {
- head = vring_add_indirect(vq, sg, out, in);
- if (head != vq->vring.num)
- goto add_head;
- }
-
BUG_ON(out + in > vq->vring.num);
BUG_ON(out + in == 0);
+ /* If the host supports indirect descriptor tables, consider it. */
+ if (vq->indirect) {
+ bool try_indirect;
+
+ /* We tweak the threshold automatically. */
+ adjust_threshold(vq, out, in);
+
+ /* If we can't fit any at all, fall through. */
+ if (vq->num_free == 0)
+ try_indirect = false;
+ else if (out + in > vq->num_free)
+ try_indirect = true;
+ else
+ try_indirect = (out + in > vq->indirect_threshold);
+
+ if (try_indirect) {
+ head = vring_add_indirect(vq, sg, out, in);
+ if (head != vq->vring.num)
+ goto add_head;
+ }
+ }
+
if (vq->num_free < out + in) {
pr_debug("Can't add buf len %i - avail = %i\n",
out + in, vq->num_free);
@@ -391,6 +434,7 @@ struct virtqueue *vring_new_virtqueue(un
#endif
vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);
+ vq->indirect_threshold = num;
/* No callback? Tell other side not to bother us. */
if (!callback)
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