Write issue taskq shouldn't be dynamic

This is as much an upstream compatibility as it's a bit of a performance gain. The illumos taskq implemention doesn't allow a TASKQ_THREADS_CPU_PCT type to be dynamic and in fact enforces as much with an ASSERT. As to performance, if this taskq is dynamic, it can cause excessive contention on tq_lock as the threads are created and destroyed because it can see bursts of many thousands of tasks in a short time, particularly in heavy high-concurrency zvol write workloads. Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Tim Chase <[email protected]> Closes openzfs#5236
behlendorf · Feb 2, 2017 · 115b8ef · 115b8ef
1 parent 31b3a14
commit 115b8ef
Showing 1 changed file with 2 additions and 1 deletion.
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
@@ -845,7 +845,7 @@ spa_taskqs_init(spa_t *spa, zio_type_t t, zio_taskq_type_t q)
 	uint_t count = ztip->zti_count;
 	spa_taskqs_t *tqs = &spa->spa_zio_taskq[t][q];
 	char name[32];
-	uint_t i, flags = TASKQ_DYNAMIC;
+	uint_t i, flags = 0;
 	boolean_t batch = B_FALSE;
 
 	if (mode == ZTI_MODE_NULL) {
@@ -863,6 +863,7 @@ spa_taskqs_init(spa_t *spa, zio_type_t t, zio_taskq_type_t q)
 	case ZTI_MODE_FIXED:
 		ASSERT3U(value, >=, 1);
 		value = MAX(value, 1);
+		flags |= TASKQ_DYNAMIC;
 		break;
 
 	case ZTI_MODE_BATCH: