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 openzfs#2572
ryao · Aug 11, 2014 · 50b25b2 · 50b25b2
1 parent ab6f407
commit 50b25b2
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Showing 2 changed files with 5 additions and 6 deletions.
diff --git a/include/sys/vdev_impl.h b/include/sys/vdev_impl.h
@@ -118,6 +118,7 @@ struct vdev_queue {
 	hrtime_t	vq_io_complete_ts; /* time last i/o completed */
 	hrtime_t	vq_io_delta_ts;
 	list_t		vq_io_list;
+	zio_t		vq_io_search; /* used as local for stack reduction */
 	kmutex_t	vq_lock;
 };
 

diff --git 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 @@ vdev_queue_io_to_issue(vdev_queue_t *vq)
 	 * 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);