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 <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #2572

diff --git a/include/sys/vdev_impl.h b/include/sys/vdev_impl.h
index 16cf4d77071ab8bee3caa89e3fab8634ccb568f5..16fcaa0422784139873f819aed3ca02f3a8396b5 100644 (file)
--- 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
index 0dc733efc3e43ba97b7de15acfc23cec92dbb824..e2758d1c41f5c63ba3ab39f4a7cf611690f6c51b 100644 (file)
--- 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);