Avoid dynamic allocation of 'search zio'

As part of commit e8b96c6 the search zio used by the
vdev_queue_io_to_issue() function was moved to the heap
to minimize stack usage.  Functionally this is fine, but
to maximize performance it's best to minimize the number
of dynamic allocations.

To avoid this allocation temporary space for the search
zio has been reserved in the vdev_queue structure.  All
access must be serialized through the vq_lock.

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ned Bass <[email protected]>
Closes #2572

1 files changed, 4 insertions, 6 deletions

diff --git a/module/zfs/vdev_queue.c b/module/zfs/vdev_queue.c
index 0dc733efc..e2758d1c4 100644
--- a/module/zfs/vdev_queue.c
+++ b/module/zfs/vdev_queue.c
@@ -659,7 +659,6 @@ vdev_queue_io_to_issue(vdev_queue_t *vq)
 	zio_priority_t p;
 	avl_index_t idx;
 	vdev_queue_class_t *vqc;
-	zio_t *search;
 
 again:
 	ASSERT(MUTEX_HELD(&vq->vq_lock));
@@ -678,11 +677,10 @@ again:
 	 * For FIFO queues (sync), issue the i/o with the lowest timestamp.
 	 */
 	vqc = &vq->vq_class[p];
-	search = zio_buf_alloc(sizeof (*search));
-	search->io_timestamp = 0;
-	search->io_offset = vq->vq_last_offset + 1;
-	VERIFY3P(avl_find(&vqc->vqc_queued_tree, search, &idx), ==, NULL);
-	zio_buf_free(search, sizeof (*search));
+	vq->vq_io_search.io_timestamp = 0;
+	vq->vq_io_search.io_offset = vq->vq_last_offset + 1;
+	VERIFY3P(avl_find(&vqc->vqc_queued_tree, &vq->vq_io_search,
+	    &idx), ==, NULL);
 	zio = avl_nearest(&vqc->vqc_queued_tree, idx, AVL_AFTER);
 	if (zio == NULL)
 		zio = avl_first(&vqc->vqc_queued_tree);