This allows more requests to fit in the descriptor ring.
Copying 1.7M kernel image 100 times (with sync between)
Before: totsegs = 55661 totlen = 148859962 avg. 2674
After: totsegs = 36097 totlen = 139439355 avg: 3862
Unfortunately, this coalescing is done at blk_rq_map_sg() which is too
late to be optimal: requests have already been limited to the value set
by blk_queue_max_hw_segments(). For us, that value reflects the
number of sg slots we can handle (ie. after clustering).
I suspect other drivers have the same issue. Jens?
(This patch is a one liner + temporary stats-gathering cruft for show)
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
---
drivers/block/virtio_blk.c | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff -r 35ec755dd1d6 drivers/block/virtio_blk.c
--- a/drivers/block/virtio_blk.c Wed Nov 12 22:41:12 2008 +1030
+++ b/drivers/block/virtio_blk.c Fri Nov 14 11:06:03 2008 +1030
@@ -37,6 +37,8 @@
struct virtio_blk_outhdr out_hdr;
u8 status;
};
+
+static unsigned long totlen, totsegs;
static void blk_done(struct virtqueue *vq)
{
@@ -102,7 +104,6 @@
sg_set_buf(&vblk->sg[0], &vbr->out_hdr, sizeof(vbr->out_hdr));
num = blk_rq_map_sg(q, vbr->req, vblk->sg+1);
sg_set_buf(&vblk->sg[num+1], &vbr->status, sizeof(vbr->status));
-
if (rq_data_dir(vbr->req) == WRITE) {
vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
out = 1 + num;
@@ -111,6 +112,13 @@
vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
out = 1;
in = 1 + num;
+ }
+
+ totsegs += out + in + 1;
+ {
+ unsigned int i;
+ for (i = 0; i < num+2; i++)
+ totlen += vblk->sg[i].length;
}
if (vblk->vq->vq_ops->add_buf(vblk->vq, vblk->sg, out, in, vbr)) {
@@ -176,6 +184,9 @@
geo->sectors = 1 << 5;
geo->cylinders = get_capacity(bd->bd_disk) >> 11;
}
+
+ printk("totsegs = %lu totlen = %lu\n", totsegs, totlen);
+ totsegs = totlen = 0;
return 0;
}
@@ -260,6 +271,9 @@
/* If barriers are supported, tell block layer that queue is ordered */
if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL);
+
+ /* Gather adjacent buffers to minimize sg length. */
+ queue_flag_set(QUEUE_FLAG_CLUSTER, vblk->disk->queue);
/* If disk is read-only in the host, the guest should obey */
if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
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