Optimize allocation throttling

Remove mc_lock use from metaslab_class_throttle_*().  The math there
is based on refcounts and so atomic, so the only race possible there
is between zfs_refcount_count() and zfs_refcount_add().  But in most
cases metaslab_class_throttle_reserve() is called with the allocator
lock held, which covers the race.  In cases where the lock is not
held, GANG_ALLOCATION() or METASLAB_MUST_RESERVE are set, and so we
do not use zfs_refcount_count().  And even if we assume some other
non-existing scenario, the worst that may happen from this race is
few more I/Os get to allocation earlier, that is not a problem.

Move locks and data of different allocators into different cache
lines to avoid false sharing.  Group spa_alloc_* arrays together
into single array of aligned struct spa_alloc spa_allocs.  Align
struct metaslab_class_allocator.

Reviewed-by: Paul Dagnelie <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Don Brady <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Sponsored-By: iXsystems, Inc.
Closes #12314

1 files changed, 16 insertions, 17 deletions

diff --git a/module/zfs/zio.c b/module/zfs/zio.c
index 6030b3813..76ed4fad4 100644
--- a/module/zfs/zio.c
+++ b/module/zfs/zio.c
@@ -877,8 +877,7 @@ zio_create(zio_t *pio, spa_t *spa, uint64_t txg, const blkptr_t *bp,
 		zio->io_bookmark = *zb;
 
 	if (pio != NULL) {
-		if (zio->io_metaslab_class == NULL)
-			zio->io_metaslab_class = pio->io_metaslab_class;
+		zio->io_metaslab_class = pio->io_metaslab_class;
 		if (zio->io_logical == NULL)
 			zio->io_logical = pio->io_logical;
 		if (zio->io_child_type == ZIO_CHILD_GANG)
@@ -3380,9 +3379,9 @@ zio_io_to_allocate(spa_t *spa, int allocator)
 {
 	zio_t *zio;
 
-	ASSERT(MUTEX_HELD(&spa->spa_alloc_locks[allocator]));
+	ASSERT(MUTEX_HELD(&spa->spa_allocs[allocator].spaa_lock));
 
-	zio = avl_first(&spa->spa_alloc_trees[allocator]);
+	zio = avl_first(&spa->spa_allocs[allocator].spaa_tree);
 	if (zio == NULL)
 		return (NULL);
 
@@ -3394,11 +3393,11 @@ zio_io_to_allocate(spa_t *spa, int allocator)
 	 */
 	ASSERT3U(zio->io_allocator, ==, allocator);
 	if (!metaslab_class_throttle_reserve(zio->io_metaslab_class,
-	    zio->io_prop.zp_copies, zio->io_allocator, zio, 0)) {
+	    zio->io_prop.zp_copies, allocator, zio, 0)) {
 		return (NULL);
 	}
 
-	avl_remove(&spa->spa_alloc_trees[allocator], zio);
+	avl_remove(&spa->spa_allocs[allocator].spaa_tree, zio);
 	ASSERT3U(zio->io_stage, <, ZIO_STAGE_DVA_ALLOCATE);
 
 	return (zio);
@@ -3422,8 +3421,8 @@ zio_dva_throttle(zio_t *zio)
 		return (zio);
 	}
 
+	ASSERT(zio->io_type == ZIO_TYPE_WRITE);
 	ASSERT(zio->io_child_type > ZIO_CHILD_GANG);
-
 	ASSERT3U(zio->io_queued_timestamp, >, 0);
 	ASSERT(zio->io_stage == ZIO_STAGE_DVA_THROTTLE);
 
@@ -3435,14 +3434,14 @@ zio_dva_throttle(zio_t *zio)
 	 * into 2^20 block regions, and then hash based on the objset, object,
 	 * level, and region to accomplish both of these goals.
 	 */
-	zio->io_allocator = cityhash4(bm->zb_objset, bm->zb_object,
+	int allocator = (uint_t)cityhash4(bm->zb_objset, bm->zb_object,
 	    bm->zb_level, bm->zb_blkid >> 20) % spa->spa_alloc_count;
-	mutex_enter(&spa->spa_alloc_locks[zio->io_allocator]);
-	ASSERT(zio->io_type == ZIO_TYPE_WRITE);
+	zio->io_allocator = allocator;
 	zio->io_metaslab_class = mc;
-	avl_add(&spa->spa_alloc_trees[zio->io_allocator], zio);
-	nio = zio_io_to_allocate(spa, zio->io_allocator);
-	mutex_exit(&spa->spa_alloc_locks[zio->io_allocator]);
+	mutex_enter(&spa->spa_allocs[allocator].spaa_lock);
+	avl_add(&spa->spa_allocs[allocator].spaa_tree, zio);
+	nio = zio_io_to_allocate(spa, allocator);
+	mutex_exit(&spa->spa_allocs[allocator].spaa_lock);
 	return (nio);
 }
 
@@ -3451,9 +3450,9 @@ zio_allocate_dispatch(spa_t *spa, int allocator)
 {
 	zio_t *zio;
 
-	mutex_enter(&spa->spa_alloc_locks[allocator]);
+	mutex_enter(&spa->spa_allocs[allocator].spaa_lock);
 	zio = zio_io_to_allocate(spa, allocator);
-	mutex_exit(&spa->spa_alloc_locks[allocator]);
+	mutex_exit(&spa->spa_allocs[allocator].spaa_lock);
 	if (zio == NULL)
 		return;
 
@@ -3643,8 +3642,8 @@ zio_alloc_zil(spa_t *spa, objset_t *os, uint64_t txg, blkptr_t *new_bp,
 	 * some parallelism.
 	 */
 	int flags = METASLAB_FASTWRITE | METASLAB_ZIL;
-	int allocator = cityhash4(0, 0, 0, os->os_dsl_dataset->ds_object) %
-	    spa->spa_alloc_count;
+	int allocator = (uint_t)cityhash4(0, 0, 0,
+	    os->os_dsl_dataset->ds_object) % spa->spa_alloc_count;
 	error = metaslab_alloc(spa, spa_log_class(spa), size, new_bp, 1,
 	    txg, NULL, flags, &io_alloc_list, NULL, allocator);
 	*slog = (error == 0);