Implement parallel dbuf eviction

In the previous code, dbuf_evict_thread() would called dbuf_evict_one()
in a look while dbuf_cache_above_lowater().

dbuf_evict_one() would select a random sublist from the dbuf cache,
then walk it from the tail forward, attempting to acquire the lock on
each object until it succeeded, then evict that object and return.

As the name suggests, it would evict only a single object from the
cache. However, evicting one object is not likely to bring us below the
desired low water mark, so dbuf_evict_one() will be called again, where
it will loop over all of the same busy objects again, until it founds
one it can evict.

This has been replaced with dbuf_evict_many() which takes a specific
sublist as a parameter, as well as a desired amount of data to evict.
It then walks the sublist from the tail forward, evicting what it can
until the number of bytes evicted satisfies the input parameter or
the head of the sublist is reached.

The dbuf_evict_thread now runs is parallel as well, allowing it to
keep up with demand more easily. For the dbuf cache, if the single
thread was not able to keep up, ZFS would shift the work of evicting
some items to each incoming I/O thread. While that is still the case
it should be seen much less often now that dbuf_evict is more efficient
and no longer bottlenecked to a single thread.

Sponsored-by: Expensify, Inc.
Sponsored-by: Klara, Inc.
Co-authored-by: Allan Jude <[email protected]>
Signed-off-by: Alexander Stetsenko <[email protected]>
Signed-off-by: Allan Jude <[email protected]>

man/man4/zfs.4

@@ -16,7 +16,7 @@
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
 .\"
-.Dd June 27, 2024
+.Dd August 28, 2024
 .Dt ZFS 4
 .Os
 .
@@ -73,6 +73,16 @@ When set to
 .Sy 0
 the array is dynamically sized based on total system memory.
 .
+.It Sy dbuf_evict_parallel Ns = Ns Sy 0 Pq uint
+When set to 1, ZFS will use up to
+.Sy dbuf_evict_threads
+threads to evict dbuf data in parallel, improving the responsiveness
+of ZFS to memory pressure.
+.
+.It Sy dbuf_evict_threads Ns = Ns Sy 0 Pq uint
+Sets the maximum number of dbuf eviction threads to be used.
+When set to 0, ZFS uses half the available CPUs or 16, whichever is less.
+.
 .It Sy dmu_object_alloc_chunk_shift Ns = Ns Sy 7 Po 128 Pc Pq uint
 dnode slots allocated in a single operation as a power of 2.
 The default value minimizes lock contention for the bulk operation performed.

module/zfs/dbuf.c

@@ -183,6 +183,7 @@ static void dbuf_sync_leaf_verify_bonus_dnode(dbuf_dirty_record_t *dr);
  */
 static kmem_cache_t *dbuf_kmem_cache;
 static taskq_t *dbu_evict_taskq;
+static taskq_t *dbuf_evict_taskq;
 
 static kthread_t *dbuf_cache_evict_thread;
 static kmutex_t dbuf_evict_lock;
@@ -237,6 +238,20 @@ static uint_t dbuf_metadata_cache_shift = 6;
 /* Set the dbuf hash mutex count as log2 shift (dynamic by default) */
 static uint_t dbuf_mutex_cache_shift = 0;
 
+/*
+ * Number of dbuf_evict threads
+ */
+static uint_t dbuf_evict_threads = 0;
+
+/*
+ * The minimum number of bytes we can evict at once is a block size.
+ * So, SPA_MAXBLOCKSIZE is a reasonable minimal value per an eviction task.
+ * We use this value to compute a scaling factor for the eviction tasks.
+ */
+#define	DBUF_MIN_EVICT_PERTASK_SHIFT	(SPA_MAXBLOCKSHIFT)
+
+static uint_t dbuf_evict_parallel = 0;
+
 static unsigned long dbuf_cache_target_bytes(void);
 static unsigned long dbuf_metadata_cache_target_bytes(void);
 
@@ -762,26 +777,47 @@ dbuf_cache_above_lowater(void)
 }
 
 /*
- * Evict the oldest eligible dbuf from the dbuf cache.
+ * Evict the oldest eligible dbufs from the dbuf cache.
+ * Use the multilist sublist (mls) with the provided index #idx.
  */
 static void
-dbuf_evict_one(void)
+dbuf_evict_many(uint64_t bytes, unsigned int idx)
 {
-	int idx = multilist_get_random_index(&dbuf_caches[DB_DBUF_CACHE].cache);
+	int64_t evicted = 0;
+	dmu_buf_impl_t *marker = kmem_cache_alloc(dbuf_kmem_cache, KM_SLEEP);
+	marker->db_objset = NULL;
+
+	ASSERT3U(idx, <, multilist_get_num_sublists(
+	    &dbuf_caches[DB_DBUF_CACHE].cache));
+
 	multilist_sublist_t *mls = multilist_sublist_lock_idx(
 	    &dbuf_caches[DB_DBUF_CACHE].cache, idx);
 
 	ASSERT(!MUTEX_HELD(&dbuf_evict_lock));
 
 	dmu_buf_impl_t *db = multilist_sublist_tail(mls);
-	while (db != NULL && mutex_tryenter(&db->db_mtx) == 0) {
-		db = multilist_sublist_prev(mls, db);
-	}
+	multilist_sublist_insert_after(mls, db, marker);
+
+	while (db != NULL && evicted < bytes) {
+		int skip = 0;
+		while (db != NULL && (db->db_objset == NULL ||
+		    mutex_tryenter(&db->db_mtx) == 0)) {
+			db = multilist_sublist_prev(mls, db);
+			if (skip == 0)
+				skip = 1;
+		}
 
-	DTRACE_PROBE2(dbuf__evict__one, dmu_buf_impl_t *, db,
-	    multilist_sublist_t *, mls);
+		if (db == NULL)
+			break;
+
+		if (skip) {
+			multilist_sublist_remove(mls, marker);
+			multilist_sublist_insert_before(mls, db, marker);
+		}
+
+		DTRACE_PROBE2(dbuf__evict__one, dmu_buf_impl_t *, db,
+		    multilist_sublist_t *, mls);
 
-	if (db != NULL) {
 		multilist_sublist_remove(mls, db);
 		multilist_sublist_unlock(mls);
 		uint64_t size = db->db.db_size;
@@ -797,9 +833,121 @@ dbuf_evict_one(void)
 		db->db_caching_status = DB_NO_CACHE;
 		dbuf_destroy(db);
 		DBUF_STAT_BUMP(cache_total_evicts);
-	} else {
-		multilist_sublist_unlock(mls);
+		evicted += size + usize;
+
+		mls = multilist_sublist_lock_idx(
+		    &dbuf_caches[DB_DBUF_CACHE].cache, idx);
+		db = multilist_sublist_prev(mls, marker);
 	}
+
+	multilist_sublist_remove(mls, marker);
+	multilist_sublist_unlock(mls);
+	kmem_cache_free(dbuf_kmem_cache, marker);
+}
+
+typedef struct evict_arg {
+	taskq_ent_t	tqe;
+	unsigned	idx;
+	uint64_t	bytes;
+} evict_arg_t;
+
+static void
+dbuf_evict_task(void *arg)
+{
+	evict_arg_t *eva = arg;
+	dbuf_evict_many(eva->bytes, eva->idx);
+}
+
+static void
+dbuf_evict(void)
+{
+	int64_t bytes = (zfs_refcount_count(&dbuf_caches[DB_DBUF_CACHE].size) -
+	    dbuf_cache_lowater_bytes());
+
+	if (bytes <= 0)
+		return;
+
+	unsigned idx = multilist_get_random_index(
+	    &dbuf_caches[DB_DBUF_CACHE].cache);
+
+	if (!dbuf_evict_parallel)
+		return (dbuf_evict_many(bytes, idx));
+
+	/*
+	 * Go to the parallel eviction.
+	 */
+	unsigned int num_sublists = multilist_get_num_sublists(
+	    &dbuf_caches[DB_DBUF_CACHE].cache);
+	evict_arg_t *evarg = kmem_zalloc(sizeof (*evarg) * num_sublists,
+	    KM_SLEEP);
+	/*
+	 *  How we scale
+	 *
+	 *  Example 1, # of chunks less than # of tasks.
+	 *  We have:
+	 *  - 4 tasks
+	 *  - 3 chunks
+	 *  - 3 full col
+	 *  - 0 low cols.
+	 *
+	 *  The first low col index is 3.
+	 *  The tasks #0-#2 evict 1 chunk each.
+	 *
+	 *    0 | 1 | 2 | 3 |
+	 *  +===+===+===+===+
+	 *  | x | x | x |   |
+	 *  +---+---+---+---+
+	 *
+	 *  Example 2, # of chunks more than # of tasks.
+	 *  We have:
+	 *  - 4 tasks
+	 *  - 9 chunks
+	 *  - 1 full col
+	 *  - 3 low cols
+	 *
+	 *  The first low col index is 1.
+	 *  The task #0 evicts 3 chunks, the others evict 2 chunks each.
+	 *
+	 *    0 | 1 | 2 | 3 |
+	 *  +===+===+===+===+
+	 *  | x | x | x | x |
+	 *  +---+---+---+---+
+	 *  | x | x | x | x |
+	 *  +---+---+---+---+
+	 *  | x |   |   |   |
+	 *  +---+---+---+---+
+	 */
+
+	/*
+	 * Compute number of tasks to run (n), first low col index (k),
+	 * normal and low bytes per task.
+	 */
+	uint64_t nchunks = ((bytes - 1) >> DBUF_MIN_EVICT_PERTASK_SHIFT) + 1;
+	unsigned n = nchunks < num_sublists ? nchunks : num_sublists;
+	uint64_t fullrows = nchunks / n;
+	unsigned lastrowcols = nchunks % n;
+	unsigned k = (lastrowcols ? lastrowcols : n);
+
+	uint64_t bytes_pertask_low = fullrows << DBUF_MIN_EVICT_PERTASK_SHIFT;
+	uint64_t bytes_pertask = bytes_pertask_low + (lastrowcols ?
+	    (1 << DBUF_MIN_EVICT_PERTASK_SHIFT) : 0);
+
+	for (unsigned i = 0; i < n; i++) {
+		uint64_t evict = i < k ? bytes_pertask : bytes_pertask_low;
+
+		evarg[i].idx = idx;
+		evarg[i].bytes = evict;
+
+		taskq_dispatch_ent(dbuf_evict_taskq, dbuf_evict_task,
+		    &evarg[i], 0, &evarg[i].tqe);
+
+		/* wrap idx */
+		if (++idx >= num_sublists)
+			idx = 0;
+	}
+
+	taskq_wait(dbuf_evict_taskq);
+	kmem_free(evarg, sizeof (*evarg) * num_sublists);
 }
 
 /*
@@ -833,7 +981,7 @@ dbuf_evict_thread(void *unused)
 		 * minimize lock contention.
 		 */
 		while (dbuf_cache_above_lowater() && !dbuf_evict_thread_exit) {
-			dbuf_evict_one();
+			dbuf_evict();
 		}
 
 		mutex_enter(&dbuf_evict_lock);
@@ -860,7 +1008,7 @@ dbuf_evict_notify(uint64_t size)
 	 */
 	if (size > dbuf_cache_target_bytes()) {
 		if (size > dbuf_cache_hiwater_bytes())
-			dbuf_evict_one();
+			dbuf_evict();
 		cv_signal(&dbuf_evict_cv);
 	}
 }
@@ -965,11 +1113,16 @@ dbuf_init(void)
 
 	dbuf_stats_init(h);
 
+	if (dbuf_evict_threads == 0)
+		dbuf_evict_threads = MIN(16, max_ncpus >> 1);
 	/*
 	 * All entries are queued via taskq_dispatch_ent(), so min/maxalloc
 	 * configuration is not required.
 	 */
 	dbu_evict_taskq = taskq_create("dbu_evict", 1, defclsyspri, 0, 0, 0);
+	dbuf_evict_taskq = taskq_create("dbuf_evict",
+	    MIN(dbuf_evict_threads, max_ncpus), defclsyspri,
+	    MIN(dbuf_evict_threads, max_ncpus), max_ncpus, TASKQ_PREPOPULATE);
 
 	for (dbuf_cached_state_t dcs = 0; dcs < DB_CACHE_MAX; dcs++) {
 		multilist_create(&dbuf_caches[dcs].cache,
@@ -1035,6 +1188,8 @@ dbuf_fini(void)
 
 	kmem_cache_destroy(dbuf_kmem_cache);
 	taskq_destroy(dbu_evict_taskq);
+	taskq_wait(dbuf_evict_taskq);
+	taskq_destroy(dbuf_evict_taskq);
 
 	mutex_enter(&dbuf_evict_lock);
 	dbuf_evict_thread_exit = B_TRUE;
@@ -3963,7 +4118,7 @@ dmu_buf_rele(dmu_buf_t *db, const void *tag)
  * dbuf_rele()-->dbuf_rele_and_unlock()-->dbuf_evict_notify()
  *	^						|
  *	|						|
- *	+-----dbuf_destroy()<--dbuf_evict_one()<--------+
+ *	+-----dbuf_destroy()<--dbuf_evict()<------------+
  *
  */
 void
@@ -5282,3 +5437,9 @@ ZFS_MODULE_PARAM(zfs_dbuf, dbuf_, metadata_cache_shift, UINT, ZMOD_RW,
 
 ZFS_MODULE_PARAM(zfs_dbuf, dbuf_, mutex_cache_shift, UINT, ZMOD_RD,
 	"Set size of dbuf cache mutex array as log2 shift.");
+
+ZFS_MODULE_PARAM(zfs_dbuf, dbuf_, evict_parallel, UINT, ZMOD_RW,
+	"Evict from the dbuf cache in parallel using a taskq");
+
+ZFS_MODULE_PARAM(zfs_dbuf, dbuf_, evict_threads, UINT, ZMOD_RW,
+	"Maximum number of dbuf_evict threads");