Experiment: Make dmu_free_long_range configurably yield the floor

module/zfs/dmu.c

@@ -56,6 +56,11 @@
  */
 int zfs_nopwrite_enabled = 1;
 
+/*
+ * Enable artificial yields when freeing a large range
+ */
+int zfs_dmu_free_long_range_yield = 0;
+
 const dmu_object_type_info_t dmu_ot[DMU_OT_NUMTYPES] = {
 	{	DMU_BSWAP_UINT8,	TRUE,	"unallocated"		},
 	{	DMU_BSWAP_ZAP,		TRUE,	"object directory"	},
@@ -647,6 +652,7 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 {
 	uint64_t object_size;
 	int err;
+	int chunks = 0;
 
 	if (dn == NULL)
 		return (SET_ERROR(EINVAL));
@@ -683,6 +689,20 @@ dmu_free_long_range_impl(objset_t *os, dnode_t *dn, uint64_t offset,
 		dmu_tx_commit(tx);
 
 		length -= chunk_end - chunk_begin;
+
+		/*
+		 * If we are asked to do so, occasionally wait for the next txg.
+		 * This should let other threads that want to use the disk in!
+		 */
+		if(zfs_dmu_free_long_range_yield > 0) {
+			if (chunks++ == zfs_dmu_free_long_range_yield) {
+				dsl_dir_t *dd = os->os_dsl_dataset->ds_dir;
+				if(dd != NULL) {
+					txg_wait_open(dd->dd_pool, 0);
+				}
+				chunks = 0;
+			}
+		}
 	}
 	return (0);
 }
@@ -2178,4 +2198,6 @@ MODULE_PARM_DESC(zfs_mdcomp_disable, "Disable meta data compression");
 module_param(zfs_nopwrite_enabled, int, 0644);
 MODULE_PARM_DESC(zfs_nopwrite_enabled, "Enable NOP writes");
 
+module_param(zfs_dmu_free_long_range_yield, int, 0644);
+MODULE_PARM_DESC(zfs_dmu_free_long_range_yield, "Enable extra waits when freeing long ranges");
 #endif