Merge MDEV-5657 (parallel replication) to 10.0

mysql-test/r/mysqld--help.result

@@ -867,6 +867,14 @@ The following options may be given as the first argument:
  operations that are idempotent. This means that CREATE
  TABLE is treated CREATE TABLE OR REPLACE and DROP TABLE
  is threated as DROP TABLE IF EXISTS. 
+ --slave-domain-parallel-threads=# 
+ Maximum number of parallel threads to use on slave for
+ events in a single replication domain. When using
+ multiple domains, this can be used to limit a single
+ domain from grabbing all threads and thus stalling other
+ domains. The default of 0 means to allow a domain to grab
+ as many threads as it wants, up to the value of
+ slave_parallel_threads.
  --slave-exec-mode=name 
  Modes for how replication events should be executed.
  Legal values are STRICT (default) and IDEMPOTENT. In
@@ -1275,6 +1283,7 @@ skip-show-database FALSE
 skip-slave-start FALSE
 slave-compressed-protocol FALSE
 slave-ddl-exec-mode IDEMPOTENT
+slave-domain-parallel-threads 0
 slave-exec-mode STRICT
 slave-max-allowed-packet 1073741824
 slave-net-timeout 3600

mysql-test/suite/perfschema/r/dml_setup_instruments.result

@@ -37,15 +37,15 @@ where name like 'Wait/Synch/Cond/sql/%'
 order by name limit 10;
 NAME	ENABLED	TIMED
 wait/synch/cond/sql/COND_flush_thread_cache	YES	YES
+wait/synch/cond/sql/COND_group_commit_orderer	YES	YES
 wait/synch/cond/sql/COND_manager	YES	YES
 wait/synch/cond/sql/COND_parallel_entry	YES	YES
 wait/synch/cond/sql/COND_prepare_ordered	YES	YES
 wait/synch/cond/sql/COND_queue_state	YES	YES
 wait/synch/cond/sql/COND_rpl_thread	YES	YES
 wait/synch/cond/sql/COND_rpl_thread_pool	YES	YES
+wait/synch/cond/sql/COND_rpl_thread_queue	YES	YES
 wait/synch/cond/sql/COND_server_started	YES	YES
-wait/synch/cond/sql/COND_thread_cache	YES	YES
-wait/synch/cond/sql/COND_thread_count	YES	YES
 select * from performance_schema.setup_instruments
 where name='Wait';
 select * from performance_schema.setup_instruments

mysql-test/suite/rpl/r/rpl_parallel.result

@@ -115,6 +115,7 @@ SET GLOBAL slave_parallel_threads=10;
 SET debug_sync='RESET';
 include/start_slave.inc
 *** Test that group-committed transactions on the master can replicate in parallel on the slave. ***
+SET debug_sync='RESET';
 FLUSH LOGS;
 CREATE TABLE t3 (a INT PRIMARY KEY, b INT) ENGINE=InnoDB;
 INSERT INTO t3 VALUES (1,1), (3,3), (5,5), (7,7);
@@ -141,6 +142,7 @@ INSERT INTO t3 VALUES (6, foo(16,
 ''));
 SET debug_sync='now WAIT_FOR master_queued3';
 SET debug_sync='now SIGNAL master_cont1';
+SET debug_sync='RESET';
 SELECT * FROM t3 ORDER BY a;
 a	b
 1	1
@@ -213,6 +215,9 @@ slave-bin.000003	#	Query	#	#	use `test`; INSERT INTO t3 VALUES (6, foo(16,
 slave-bin.000003	#	Xid	#	#	COMMIT /* XID */
 *** Test STOP SLAVE in parallel mode ***
 include/stop_slave.inc
+SET debug_sync='RESET';
+SET GLOBAL slave_parallel_threads=0;
+SET GLOBAL slave_parallel_threads=10;
 SET binlog_direct_non_transactional_updates=0;
 SET sql_log_bin=0;
 CALL mtr.add_suppression("Statement is unsafe because it accesses a non-transactional table after accessing a transactional table within the same transaction");
@@ -227,10 +232,15 @@ INSERT INTO t3 VALUES(21, 21);
 INSERT INTO t3 VALUES(22, 22);
 SET binlog_format=@old_format;
 BEGIN;
-INSERT INTO t2 VALUES  (21);
+INSERT INTO t2 VALUES (21);
 START SLAVE;
+SET @old_dbug= @@GLOBAL.debug_dbug;
+SET GLOBAL debug_dbug="+d,rpl_parallel_wait_for_done_trigger";
 STOP SLAVE;
+SET debug_sync='now WAIT_FOR wait_for_done_waiting';
 ROLLBACK;
+SET GLOBAL debug_dbug=@old_dbug;
+SET debug_sync='RESET';
 include/wait_for_slave_to_stop.inc
 SELECT * FROM t1 WHERE a >= 20 ORDER BY a;
 a
@@ -292,6 +302,7 @@ a	b
 32	32
 33	33
 34	34
+SET debug_sync='RESET';
 SET sql_log_bin=0;
 CALL mtr.add_suppression("Query execution was interrupted");
 CALL mtr.add_suppression("Commit failed due to failure of an earlier commit on which this one depends");
@@ -308,6 +319,7 @@ STOP SLAVE IO_THREAD;
 SELECT * FROM t3 WHERE a >= 30 ORDER BY a;
 a	b
 31	31
+SET debug_sync='RESET';
 SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 SET sql_log_bin=0;
@@ -379,6 +391,7 @@ a	b
 42	42
 43	43
 44	44
+SET debug_sync='RESET';
 SET debug_sync='now WAIT_FOR t2_query';
 SET debug_sync='now SIGNAL t2_cont';
 SET debug_sync='now WAIT_FOR t1_ready';
@@ -386,9 +399,7 @@ KILL THD_ID;
 SET debug_sync='now WAIT_FOR t2_killed';
 SET debug_sync='now SIGNAL t1_cont';
 include/wait_for_slave_sql_error.inc [errno=1317,1964]
-SELECT * FROM t3 WHERE a >= 40 ORDER BY a;
-a	b
-41	41
+SET debug_sync='RESET';
 SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 SET sql_log_bin=0;
@@ -463,6 +474,7 @@ a	b
 52	52
 53	53
 54	54
+SET debug_sync='RESET';
 SET debug_sync='now WAIT_FOR t2_query';
 SET debug_sync='now SIGNAL t2_cont';
 SET debug_sync='now WAIT_FOR t1_ready';
@@ -473,6 +485,7 @@ include/wait_for_slave_sql_error.inc [errno=1317,1964]
 SELECT * FROM t3 WHERE a >= 50 ORDER BY a;
 a	b
 51	51
+SET debug_sync='RESET';
 SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 SET sql_log_bin=0;
@@ -514,14 +527,18 @@ include/start_slave.inc
 include/stop_slave.inc
 SET GLOBAL binlog_format=@old_format;
 SET GLOBAL slave_parallel_threads=0;
-SET GLOBAL slave_parallel_threads=3;
+SET GLOBAL slave_parallel_threads=4;
 include/start_slave.inc
 *** 4. Test killing thread that is waiting to start transaction until previous transaction commits ***
 SET binlog_format=statement;
 SET gtid_domain_id=2;
+BEGIN;
+INSERT INTO t3 VALUES (70, foo(70,
+'rpl_parallel_start_waiting_for_prior SIGNAL t4_waiting', ''));
 INSERT INTO t3 VALUES (60, foo(60,
 'ha_write_row_end SIGNAL d2_query WAIT_FOR d2_cont2',
 'rpl_parallel_end_of_group SIGNAL d2_done WAIT_FOR d2_cont'));
+COMMIT;
 SET gtid_domain_id=0;
 SET debug_sync='now WAIT_FOR d2_query';
 SET gtid_domain_id=1;
@@ -540,15 +557,27 @@ INSERT INTO t3 VALUES (63, foo(63,
 'ha_write_row_end SIGNAL d0_query WAIT_FOR d0_cont2',
 'rpl_parallel_end_of_group SIGNAL d0_done WAIT_FOR d0_cont'));
 SET debug_sync='now WAIT_FOR d0_query';
+SET gtid_domain_id=3;
+BEGIN;
+INSERT INTO t3 VALUES (68, foo(68,
+'rpl_parallel_start_waiting_for_prior SIGNAL t2_waiting', ''));
+INSERT INTO t3 VALUES (69, foo(69,
+'ha_write_row_end SIGNAL d3_query WAIT_FOR d3_cont2',
+'rpl_parallel_end_of_group SIGNAL d3_done WAIT_FOR d3_cont'));
+COMMIT;
+SET gtid_domain_id=0;
+SET debug_sync='now WAIT_FOR d3_query';
 SET debug_sync='now SIGNAL d2_cont2';
 SET debug_sync='now WAIT_FOR d2_done';
 SET debug_sync='now SIGNAL d1_cont2';
 SET debug_sync='now WAIT_FOR d1_done';
 SET debug_sync='now SIGNAL d0_cont2';
 SET debug_sync='now WAIT_FOR d0_done';
+SET debug_sync='now SIGNAL d3_cont2';
+SET debug_sync='now WAIT_FOR d3_done';
 SET binlog_format=statement;
 INSERT INTO t3 VALUES (64, foo(64,
-'commit_before_prepare_ordered SIGNAL t1_waiting WAIT_FOR t1_cont', ''));
+'rpl_parallel_before_mark_start_commit SIGNAL t1_waiting WAIT_FOR t1_cont', ''));
 SET debug_sync='commit_after_release_LOCK_prepare_ordered SIGNAL master_queued2 WAIT_FOR master_cont2';
 INSERT INTO t3 VALUES (65, foo(65, '', ''));
 SET debug_sync='now WAIT_FOR master_queued2';
@@ -569,23 +598,34 @@ a	b
 65	65
 66	66
 67	67
+68	68
+69	69
+70	70
+SET debug_sync='RESET';
 SET debug_sync='now SIGNAL d0_cont';
 SET debug_sync='now WAIT_FOR t1_waiting';
+SET debug_sync='now SIGNAL d3_cont';
+SET debug_sync='now WAIT_FOR t2_waiting';
 SET debug_sync='now SIGNAL d1_cont';
 SET debug_sync='now WAIT_FOR t3_waiting';
 SET debug_sync='now SIGNAL d2_cont';
+SET debug_sync='now WAIT_FOR t4_waiting';
 KILL THD_ID;
 SET debug_sync='now WAIT_FOR t3_killed';
 SET debug_sync='now SIGNAL t1_cont';
 include/wait_for_slave_sql_error.inc [errno=1317,1927,1964]
 STOP SLAVE IO_THREAD;
-SELECT * FROM t3 WHERE a >= 60 ORDER BY a;
+SELECT * FROM t3 WHERE a >= 60 AND a != 65 ORDER BY a;
 a	b
 60	60
 61	61
 62	62
 63	63
 64	64
+68	68
+69	69
+70	70
+SET debug_sync='RESET';
 SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 SET sql_log_bin=0;
@@ -597,11 +637,11 @@ RETURN x;
 END
 ||
 SET sql_log_bin=1;
-INSERT INTO t3 VALUES (69,0);
+UPDATE t3 SET b=b+1 WHERE a=60;
 include/start_slave.inc
 SELECT * FROM t3 WHERE a >= 60 ORDER BY a;
 a	b
-60	60
+60	61
 61	61
 62	62
 63	63
@@ -609,7 +649,9 @@ a	b
 65	65
 66	66
 67	67
-69	0
+68	68
+69	69
+70	70
 SET sql_log_bin=0;
 DROP FUNCTION foo;
 CREATE FUNCTION foo(x INT, d1 VARCHAR(500), d2 VARCHAR(500))
@@ -634,37 +676,31 @@ include/start_slave.inc
 SET @old_max_queued= @@GLOBAL.slave_parallel_max_queued;
 SET GLOBAL slave_parallel_max_queued=9000;
 SET binlog_format=statement;
-INSERT INTO t3 VALUES (70, foo(0,
+INSERT INTO t3 VALUES (80, foo(0,
 'ha_write_row_end SIGNAL query_waiting WAIT_FOR query_cont', ''));
 SET debug_sync='now WAIT_FOR query_waiting';
 SET @old_dbug= @@GLOBAL.debug_dbug;
 SET GLOBAL debug_dbug="+d,rpl_parallel_wait_queue_max";
-INSERT INTO t3 VALUES (72, 0);
-SELECT * FROM t3 WHERE a >= 70 ORDER BY a;
+SELECT * FROM t3 WHERE a >= 80 ORDER BY a;
 a	b
-70	0
-71	10000
-72	0
+80	0
+81	10000
 SET debug_sync='now WAIT_FOR wait_queue_ready';
 KILL THD_ID;
 SET debug_sync='now WAIT_FOR wait_queue_killed';
 SET debug_sync='now SIGNAL query_cont';
 include/wait_for_slave_sql_error.inc [errno=1317,1927,1964]
 STOP SLAVE IO_THREAD;
-SELECT * FROM t3 WHERE a >= 70 ORDER BY a;
-a	b
-70	0
-71	10000
 SET GLOBAL debug_dbug=@old_dbug;
 SET GLOBAL slave_parallel_max_queued= @old_max_queued;
-INSERT INTO t3 VALUES (73,0);
+INSERT INTO t3 VALUES (82,0);
+SET debug_sync='RESET';
 include/start_slave.inc
-SELECT * FROM t3 WHERE a >= 70 ORDER BY a;
+SELECT * FROM t3 WHERE a >= 80 ORDER BY a;
 a	b
-70	0
-71	10000
-72	0
-73	0
+80	0
+81	10000
+82	0
 include/stop_slave.inc
 SET GLOBAL binlog_format=@old_format;
 SET GLOBAL slave_parallel_threads=0;

mysql-test/suite/sys_vars/r/slave_domain_parallel_threads_basic.result

+SET @save_slave_domain_parallel_threads= @@GLOBAL.slave_domain_parallel_threads;
+SELECT @@GLOBAL.slave_domain_parallel_threads as 'must be zero because of default';
+must be zero because of default
+0
+SELECT @@SESSION.slave_domain_parallel_threads  as 'no session var';
+ERROR HY000: Variable 'slave_domain_parallel_threads' is a GLOBAL variable
+SET GLOBAL slave_domain_parallel_threads= 0;
+SET GLOBAL slave_domain_parallel_threads= DEFAULT;
+SET GLOBAL slave_domain_parallel_threads= 10;
+SELECT @@GLOBAL.slave_domain_parallel_threads;
+@@GLOBAL.slave_domain_parallel_threads
+10
+SET GLOBAL slave_domain_parallel_threads = @save_slave_domain_parallel_threads;

mysql-test/suite/sys_vars/t/slave_domain_parallel_threads_basic.test

+--source include/not_embedded.inc
+
+SET @save_slave_domain_parallel_threads= @@GLOBAL.slave_domain_parallel_threads;
+
+SELECT @@GLOBAL.slave_domain_parallel_threads as 'must be zero because of default';
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+SELECT @@SESSION.slave_domain_parallel_threads  as 'no session var';
+
+SET GLOBAL slave_domain_parallel_threads= 0;
+SET GLOBAL slave_domain_parallel_threads= DEFAULT;
+SET GLOBAL slave_domain_parallel_threads= 10;
+SELECT @@GLOBAL.slave_domain_parallel_threads;
+
+SET GLOBAL slave_domain_parallel_threads = @save_slave_domain_parallel_threads;

mysys/mf_iocache.c

@@ -1281,10 +1281,6 @@ read_append_buffer:
     size_t transfer_len;
 
     DBUG_ASSERT(info->append_read_pos <= info->write_pos);
-    /*
-      TODO: figure out if the assert below is needed or correct.
-    */
-    DBUG_ASSERT(pos_in_file == info->end_of_file);
     copy_len=MY_MIN(Count, len_in_buff);
     memcpy(Buffer, info->append_read_pos, copy_len);
     info->append_read_pos += copy_len;

sql/log.cc

@@ -6710,13 +6710,15 @@ MYSQL_BIN_LOG::queue_for_group_commit(group_commit_entry *orig_entry)
   */
   wfc= orig_entry->thd->wait_for_commit_ptr;
   orig_entry->queued_by_other= false;
-  if (wfc && wfc->waiting_for_commit)
+  if (wfc && wfc->waitee)
   {
     mysql_mutex_lock(&wfc->LOCK_wait_commit);
     /* Do an extra check here, this time safely under lock. */
-    if (wfc->waiting_for_commit)
+    if (wfc->waitee)
     {
       PSI_stage_info old_stage;
+      wait_for_commit *loc_waitee;
+
       /*
         By setting wfc->opaque_pointer to our own entry, we mark that we are
         ready to commit, but waiting for another transaction to commit before
@@ -6727,21 +6729,20 @@ MYSQL_BIN_LOG::queue_for_group_commit(group_commit_entry *orig_entry)
         queued_by_other flag is set.
       */
       wfc->opaque_pointer= orig_entry;
+      DEBUG_SYNC(orig_entry->thd, "group_commit_waiting_for_prior");
       orig_entry->thd->ENTER_COND(&wfc->COND_wait_commit,
                                   &wfc->LOCK_wait_commit,
                                   &stage_waiting_for_prior_transaction_to_commit,
                                   &old_stage);
-      DEBUG_SYNC(orig_entry->thd, "group_commit_waiting_for_prior");
-      while (wfc->waiting_for_commit && !orig_entry->thd->check_killed())
+      while ((loc_waitee= wfc->waitee) && !orig_entry->thd->check_killed())
         mysql_cond_wait(&wfc->COND_wait_commit, &wfc->LOCK_wait_commit);
       wfc->opaque_pointer= NULL;
       DBUG_PRINT("info", ("After waiting for prior commit, queued_by_other=%d",
                  orig_entry->queued_by_other));
 
-      if (wfc->waiting_for_commit)
+      if (loc_waitee)
       {
         /* Wait terminated due to kill. */
-        wait_for_commit *loc_waitee= wfc->waitee;
         mysql_mutex_lock(&loc_waitee->LOCK_wait_commit);
         if (loc_waitee->wakeup_subsequent_commits_running ||
             orig_entry->queued_by_other)
@@ -6751,13 +6752,14 @@ MYSQL_BIN_LOG::queue_for_group_commit(group_commit_entry *orig_entry)
           do
           {
             mysql_cond_wait(&wfc->COND_wait_commit, &wfc->LOCK_wait_commit);
-          } while (wfc->waiting_for_commit);
+          } while (wfc->waitee);
         }
         else
         {
           /* We were killed, so remove us from the list of waitee. */
           wfc->remove_from_list(&loc_waitee->subsequent_commits_list);
           mysql_mutex_unlock(&loc_waitee->LOCK_wait_commit);
+          wfc->waitee= NULL;
 
           orig_entry->thd->EXIT_COND(&old_stage);
           /* Interrupted by kill. */
@@ -6773,12 +6775,11 @@ MYSQL_BIN_LOG::queue_for_group_commit(group_commit_entry *orig_entry)
     }
     else
       mysql_mutex_unlock(&wfc->LOCK_wait_commit);
-
-    if (wfc->wakeup_error)
-    {
-      my_error(ER_PRIOR_COMMIT_FAILED, MYF(0));
-      DBUG_RETURN(-1);
-    }
+  }
+  if (wfc && wfc->wakeup_error)
+  {
+    my_error(ER_PRIOR_COMMIT_FAILED, MYF(0));
+    DBUG_RETURN(-1);
   }
 
   /*
@@ -9110,7 +9111,7 @@ start_binlog_background_thread()
                                 array_elements(all_binlog_threads));
 #endif
 
-  if (mysql_thread_create(key_thread_binlog, &th, NULL,
+  if (mysql_thread_create(key_thread_binlog, &th, &connection_attrib,
                           binlog_background_thread, NULL))
     return 1;
 

sql/mysqld.cc

@@ -549,6 +549,7 @@ ulong rpl_recovery_rank=0;
 ulong stored_program_cache_size= 0;
 
 ulong opt_slave_parallel_threads= 0;
+ulong opt_slave_domain_parallel_threads= 0;
 ulong opt_binlog_commit_wait_count= 0;
 ulong opt_binlog_commit_wait_usec= 0;
 ulong opt_slave_parallel_max_queued= 131072;
@@ -982,8 +983,10 @@ PSI_cond_key key_RELAYLOG_update_cond, key_COND_wakeup_ready,
   key_COND_wait_commit;
 PSI_cond_key key_RELAYLOG_COND_queue_busy;
 PSI_cond_key key_TC_LOG_MMAP_COND_queue_busy;
-PSI_cond_key key_COND_rpl_thread, key_COND_rpl_thread_pool,
-  key_COND_parallel_entry, key_COND_prepare_ordered;
+PSI_cond_key key_COND_rpl_thread_queue, key_COND_rpl_thread,
+  key_COND_rpl_thread_pool,
+  key_COND_parallel_entry, key_COND_group_commit_orderer,
+  key_COND_prepare_ordered;
 PSI_cond_key key_COND_wait_gtid;
 
 static PSI_cond_info all_server_conds[]=
@@ -1027,8 +1030,10 @@ static PSI_cond_info all_server_conds[]=
   { &key_COND_thread_cache, "COND_thread_cache", PSI_FLAG_GLOBAL},
   { &key_COND_flush_thread_cache, "COND_flush_thread_cache", PSI_FLAG_GLOBAL},
   { &key_COND_rpl_thread, "COND_rpl_thread", 0},
+  { &key_COND_rpl_thread_queue, "COND_rpl_thread_queue", 0},
   { &key_COND_rpl_thread_pool, "COND_rpl_thread_pool", 0},
   { &key_COND_parallel_entry, "COND_parallel_entry", 0},
+  { &key_COND_group_commit_orderer, "COND_group_commit_orderer", 0},
   { &key_COND_prepare_ordered, "COND_prepare_ordered", 0},
   { &key_COND_wait_gtid, "COND_wait_gtid", 0}
 };
@@ -9436,7 +9441,7 @@ PSI_stage_info stage_binlog_waiting_background_tasks= { 0, "Waiting for backgrou
 PSI_stage_info stage_binlog_processing_checkpoint_notify= { 0, "Processing binlog checkpoint notification", 0};
 PSI_stage_info stage_binlog_stopping_background_thread= { 0, "Stopping binlog background thread", 0};
 PSI_stage_info stage_waiting_for_work_from_sql_thread= { 0, "Waiting for work from SQL thread", 0};
-PSI_stage_info stage_waiting_for_prior_transaction_to_commit= { 0, "Waiting for prior transaction to commit", 0};
+PSI_stage_info stage_waiting_for_prior_transaction_to_commit= { 0, "Waiting for prior transaction to start commit before starting next transaction", 0};
 PSI_stage_info stage_waiting_for_room_in_worker_thread= { 0, "Waiting for room in worker thread event queue", 0};
 PSI_stage_info stage_master_gtid_wait_primary= { 0, "Waiting in MASTER_GTID_WAIT() (primary waiter)", 0};
 PSI_stage_info stage_master_gtid_wait= { 0, "Waiting in MASTER_GTID_WAIT()", 0};

sql/mysqld.h

@@ -180,6 +180,7 @@ extern ulong opt_binlog_rows_event_max_size;
 extern ulong rpl_recovery_rank, thread_cache_size;
 extern ulong stored_program_cache_size;
 extern ulong opt_slave_parallel_threads;
+extern ulong opt_slave_domain_parallel_threads;
 extern ulong opt_slave_parallel_max_queued;
 extern ulong opt_binlog_commit_wait_count;
 extern ulong opt_binlog_commit_wait_usec;
@@ -295,8 +296,9 @@ extern PSI_cond_key key_RELAYLOG_update_cond, key_COND_wakeup_ready,
   key_COND_wait_commit;
 extern PSI_cond_key key_RELAYLOG_COND_queue_busy;
 extern PSI_cond_key key_TC_LOG_MMAP_COND_queue_busy;
-extern PSI_cond_key key_COND_rpl_thread, key_COND_rpl_thread_pool,
-  key_COND_parallel_entry;
+extern PSI_cond_key key_COND_rpl_thread, key_COND_rpl_thread_queue,
+  key_COND_rpl_thread_pool,
+  key_COND_parallel_entry, key_COND_group_commit_orderer;
 extern PSI_cond_key key_COND_wait_gtid;
 
 extern PSI_thread_key key_thread_bootstrap, key_thread_delayed_insert,

sql/rpl_parallel.h

@@ -9,16 +9,66 @@ struct rpl_parallel_entry;
 struct rpl_parallel_thread_pool;
 
 class Relay_log_info;
+
+
+/*
+  Structure used to keep track of the parallel replication of a batch of
+  event-groups that group-committed together on the master.
+
+  It is used to ensure that every event group in one batch has reached the
+  commit stage before the next batch starts executing.
+
+  Note the lifetime of this structure:
+
+   - It is allocated when the first event in a new batch of group commits
+     is queued, from the free list rpl_parallel_entry::gco_free_list.
+
+   - The gco for the batch currently being queued is owned by
+     rpl_parallel_entry::current_gco. The gco for a previous batch that has
+     been fully queued is owned by the gco->prev_gco pointer of the gco for
+     the following batch.
+
+   - The worker thread waits on gco->COND_group_commit_orderer for
+     rpl_parallel_entry::count_committing_event_groups to reach wait_count
+     before starting; the first waiter links the gco into the next_gco
+     pointer of the gco of the previous batch for signalling.
+
+   - When an event group reaches the commit stage, it signals the
+     COND_group_commit_orderer if its gco->next_gco pointer is non-NULL and
+     rpl_parallel_entry::count_committing_event_groups has reached
+     gco->next_gco->wait_count.
+
+   - When gco->wait_count is reached for a worker and the wait completes,
+     the worker frees gco->prev_gco; at this point it is guaranteed not to
+     be needed any longer.
+*/
+struct group_commit_orderer {
+  /* Wakeup condition, used with rpl_parallel_entry::LOCK_parallel_entry. */
+  mysql_cond_t COND_group_commit_orderer;
+  uint64 wait_count;
+  group_commit_orderer *prev_gco;
+  group_commit_orderer *next_gco;
+  bool installed;
+};
+
+
 struct rpl_parallel_thread {
   bool delay_start;
   bool running;
   bool stop;
   mysql_mutex_t LOCK_rpl_thread;
   mysql_cond_t COND_rpl_thread;
+  mysql_cond_t COND_rpl_thread_queue;
   struct rpl_parallel_thread *next;             /* For free list. */
   struct rpl_parallel_thread_pool *pool;
   THD *thd;
-  struct rpl_parallel_entry *current_entry;
+  /*
+    Who owns the thread, if any (it's a pointer into the
+    rpl_parallel_entry::rpl_threads array.
+  */
+  struct rpl_parallel_thread **current_owner;
+  /* The rpl_parallel_entry of the owner. */
+  rpl_parallel_entry *current_entry;
   struct queued_event {
     queued_event *next;
     Log_event *ev;
@@ -31,6 +81,9 @@ struct rpl_parallel_thread {
     size_t event_size;
   } *event_queue, *last_in_queue;
   uint64 queued_size;
+  queued_event *qev_free_list;
+  rpl_group_info *rgi_free_list;
+  group_commit_orderer *gco_free_list;
 
   void enqueue(queued_event *qev)
   {
@@ -42,15 +95,25 @@ struct rpl_parallel_thread {
     queued_size+= qev->event_size;
   }
 
-  void dequeue(queued_event *list)
+  void dequeue1(queued_event *list)
   {
-    queued_event *tmp;
-
     DBUG_ASSERT(list == event_queue);
     event_queue= last_in_queue= NULL;
-    for (tmp= list; tmp; tmp= tmp->next)
-      queued_size-= tmp->event_size;
   }
+
+  void dequeue2(size_t dequeue_size)
+  {
+    queued_size-= dequeue_size;
+  }
+
+  queued_event *get_qev(Log_event *ev, ulonglong event_size,
+                        Relay_log_info *rli);
+  void free_qev(queued_event *qev);
+  rpl_group_info *get_rgi(Relay_log_info *rli, Gtid_log_event *gtid_ev,
+                          rpl_parallel_entry *e);
+  void free_rgi(rpl_group_info *rgi);
+  group_commit_orderer *get_gco(uint64 wait_count, group_commit_orderer *prev);
+  void free_gco(group_commit_orderer *gco);
 };
 
 
@@ -66,14 +129,16 @@ struct rpl_parallel_thread_pool {
   rpl_parallel_thread_pool();
   int init(uint32 size);
   void destroy();
-  struct rpl_parallel_thread *get_thread(rpl_parallel_entry *entry);
+  struct rpl_parallel_thread *get_thread(rpl_parallel_thread **owner,
+                                         rpl_parallel_entry *entry);
+  void release_thread(rpl_parallel_thread *rpt);
 };
 
 
 struct rpl_parallel_entry {
+  mysql_mutex_t LOCK_parallel_entry;
+  mysql_cond_t COND_parallel_entry;
   uint32 domain_id;
-  uint32 last_server_id;
-  uint64 last_seq_no;
   uint64 last_commit_id;
   bool active;
   /*
@@ -82,15 +147,41 @@ struct rpl_parallel_entry {
     waiting for event groups to complete.
   */
   bool force_abort;
+  /*
+   At STOP SLAVE (force_abort=true), we do not want to process all events in
+   the queue (which could unnecessarily delay stop, if a lot of events happen
+   to be queued). The stop_count provides a safe point at which to stop, so
+   that everything before becomes committed and nothing after does. The value
+   corresponds to group_commit_orderer::wait_count; if wait_count is less than
+   or equal to stop_count, we execute the associated event group, else we
+   skip it (and all following) and stop.
+  */
+  uint64 stop_count;
 
-  rpl_parallel_thread *rpl_thread;
+  /*
+    Cyclic array recording the last rpl_thread_max worker threads that we
+    queued event for. This is used to limit how many workers a single domain
+    can occupy (--slave-domain-parallel-threads).
+
+    Note that workers are never explicitly deleted from the array. Instead,
+    we need to check (under LOCK_rpl_thread) that the thread still belongs
+    to us before re-using (rpl_thread::current_owner).
+  */
+  rpl_parallel_thread **rpl_threads;
+  uint32 rpl_thread_max;
+  uint32 rpl_thread_idx;
   /*
     The sub_id of the last transaction to commit within this domain_id.
     Must be accessed under LOCK_parallel_entry protection.
+
+    Event groups commit in order, so the rpl_group_info for an event group
+    will be alive (at least) as long as
+    rpl_grou_info::gtid_sub_id > last_committed_sub_id. This can be used to
+    safely refer back to previous event groups if they are still executing,
+    and ignore them if they completed, without requiring explicit
+    synchronisation between the threads.
   */
   uint64 last_committed_sub_id;
-  mysql_mutex_t LOCK_parallel_entry;
-  mysql_cond_t COND_parallel_entry;
   /*
     The sub_id of the last event group in this replication domain that was
     queued for execution by a worker thread.
@@ -98,14 +189,29 @@ struct rpl_parallel_entry {
   uint64 current_sub_id;
   rpl_group_info *current_group_info;
   /*
-    The sub_id of the last event group in the previous batch of group-committed
-    transactions.
-
-    When we spawn parallel worker threads for the next group-committed batch,
-    they first need to wait for this sub_id to be committed before it is safe
-    to start executing them.
+    If we get an error in some event group, we set the sub_id of that event
+    group here. Then later event groups (with higher sub_id) can know not to
+    try to start (event groups that already started will be rolled back when
+    wait_for_prior_commit() returns error).
+    The value is ULONGLONG_MAX when no error occured.
+  */
+  uint64 stop_on_error_sub_id;
+  /* Total count of event groups queued so far. */
+  uint64 count_queued_event_groups;
+  /*
+    Count of event groups that have started (but not necessarily completed)
+    the commit phase. We use this to know when every event group in a previous
+    batch of master group commits have started committing on the slave, so
+    that it is safe to start executing the events in the following batch.
   */
-  uint64 prev_groupcommit_sub_id;
+  uint64 count_committing_event_groups;
+  /* The group_commit_orderer object for the events currently being queued. */
+  group_commit_orderer *current_gco;
+
+  rpl_parallel_thread * choose_thread(Relay_log_info *rli, bool *did_enter_cond,
+                                      PSI_stage_info *old_stage, bool reuse);
+  group_commit_orderer *get_gco();
+  void free_gco(group_commit_orderer *gco);
 };
 struct rpl_parallel {
   HASH domain_hash;
@@ -116,7 +222,7 @@ struct rpl_parallel {
   ~rpl_parallel();
   void reset();
   rpl_parallel_entry *find(uint32 domain_id);
-  void wait_for_done();
+  void wait_for_done(THD *thd);
   bool workers_idle();
   bool do_event(rpl_group_info *serial_rgi, Log_event *ev,
                 ulonglong event_size);

sql/rpl_rli.cc

@@ -1479,14 +1479,27 @@ end:
 }
 
 
-rpl_group_info::rpl_group_info(Relay_log_info *rli_)
-  : rli(rli_), thd(0), gtid_sub_id(0), wait_commit_sub_id(0),
-    wait_commit_group_info(0), wait_start_sub_id(0), parallel_entry(0),
-    deferred_events(NULL), m_annotate_event(0), tables_to_lock(0),
-    tables_to_lock_count(0), trans_retries(0), last_event_start_time(0),
-    is_parallel_exec(false), is_error(false),
-    row_stmt_start_timestamp(0), long_find_row_note_printed(false)
+void
+rpl_group_info::reinit(Relay_log_info *rli)
+{
+  this->rli= rli;
+  tables_to_lock= NULL;
+  tables_to_lock_count= 0;
+  trans_retries= 0;
+  last_event_start_time= 0;
+  is_error= false;
+  row_stmt_start_timestamp= 0;
+  long_find_row_note_printed= false;
+  did_mark_start_commit= false;
+  commit_orderer.reinit();
+}
+
+rpl_group_info::rpl_group_info(Relay_log_info *rli)
+  : thd(0), gtid_sub_id(0), wait_commit_sub_id(0),
+    wait_commit_group_info(0), parallel_entry(0),
+    deferred_events(NULL), m_annotate_event(0), is_parallel_exec(false)
 {
+  reinit(rli);
   bzero(&current_gtid, sizeof(current_gtid));
   mysql_mutex_init(key_rpl_group_info_sleep_lock, &sleep_lock,
                    MY_MUTEX_INIT_FAST);
@@ -1710,4 +1723,40 @@ void rpl_group_info::slave_close_thread_tables(THD *thd)
 }
 
 
+
+static void
+mark_start_commit_inner(rpl_parallel_entry *e, group_commit_orderer *gco)
+{
+  uint64 count= ++e->count_committing_event_groups;
+  if (gco->next_gco && gco->next_gco->wait_count == count)
+    mysql_cond_broadcast(&gco->next_gco->COND_group_commit_orderer);
+}
+
+
+void
+rpl_group_info::mark_start_commit_no_lock()
+{
+  if (did_mark_start_commit)
+    return;
+  mark_start_commit_inner(parallel_entry, gco);
+  did_mark_start_commit= true;
+}
+
+
+void
+rpl_group_info::mark_start_commit()
+{
+  rpl_parallel_entry *e;
+
+  if (did_mark_start_commit)
+    return;
+
+  e= this->parallel_entry;
+  mysql_mutex_lock(&e->LOCK_parallel_entry);
+  mark_start_commit_inner(e, gco);
+  mysql_mutex_unlock(&e->LOCK_parallel_entry);
+  did_mark_start_commit= true;
+}
+
+
 #endif

sql/rpl_rli.h

@@ -481,6 +481,7 @@ private:
 
 struct rpl_group_info
 {
+  rpl_group_info *next;             /* For free list in rpl_parallel_thread */
   Relay_log_info *rli;
   THD *thd;
   /*
@@ -510,14 +511,15 @@ struct rpl_group_info
   uint64 wait_commit_sub_id;
   rpl_group_info *wait_commit_group_info;
   /*
-    If non-zero, the event group must wait for this sub_id to be committed
-    before the execution of the event group is allowed to start.
+    This holds a pointer to a struct that keeps track of the need to wait
+    for the previous batch of event groups to reach the commit stage, before
+    this batch can start to execute.
 
     (When we execute in parallel the transactions that group committed
     together on the master, we still need to wait for any prior transactions
-    to have commtted).
+    to have reached the commit stage).
   */
-  uint64 wait_start_sub_id;
+  group_commit_orderer *gco;
 
   struct rpl_parallel_entry *parallel_entry;
 
@@ -567,18 +569,22 @@ struct rpl_group_info
   char future_event_master_log_name[FN_REFLEN];
   bool is_parallel_exec;
   bool is_error;
+  /*
+    Set true when we signalled that we reach the commit phase. Used to avoid
+    counting one event group twice.
+  */
+  bool did_mark_start_commit;
 
-private:
   /*
     Runtime state for printing a note when slave is taking
     too long while processing a row event.
    */
   time_t row_stmt_start_timestamp;
   bool long_find_row_note_printed;
-public:
 
   rpl_group_info(Relay_log_info *rli_);
   ~rpl_group_info();
+  void reinit(Relay_log_info *rli);
 
   /* 
      Returns true if the argument event resides in the containter;
@@ -661,6 +667,8 @@ public:
   void clear_tables_to_lock();
   void cleanup_context(THD *, bool);
   void slave_close_thread_tables(THD *);
+  void mark_start_commit_no_lock();
+  void mark_start_commit();
 
   time_t get_row_stmt_start_timestamp()
   {

sql/slave.cc

@@ -331,7 +331,7 @@ run_slave_init_thread()
   pthread_t th;
 
   slave_init_thread_running= true;
-  if (mysql_thread_create(key_thread_slave_init, &th, NULL,
+  if (mysql_thread_create(key_thread_slave_init, &th, &connection_attrib,
                           handle_slave_init, NULL))
   {
     sql_print_error("Failed to create thread while initialising slave");
@@ -4542,7 +4542,7 @@ log '%s' at position %s, relay log '%s' position: %s%s", RPL_LOG_NAME,
   }
 
   if (opt_slave_parallel_threads > 0)
-    rli->parallel.wait_for_done();
+    rli->parallel.wait_for_done(thd);
 
   /* Thread stopped. Print the current replication position to the log */
   {
@@ -4568,7 +4568,7 @@ log '%s' at position %s, relay log '%s' position: %s%s", RPL_LOG_NAME,
     get the correct position printed.)
   */
   if (opt_slave_parallel_threads > 0)
-    rli->parallel.wait_for_done();
+    rli->parallel.wait_for_done(thd);
 
   /*
     Some events set some playgrounds, which won't be cleared because thread

sql/sql_class.cc

@@ -6118,14 +6118,23 @@ bool THD::rgi_have_temporary_tables()
 }
 
 
+void
+wait_for_commit::reinit()
+{
+  subsequent_commits_list= NULL;
+  next_subsequent_commit= NULL;
+  waitee= NULL;
+  opaque_pointer= NULL;
+  wakeup_error= 0;
+  wakeup_subsequent_commits_running= false;
+}
+
+
 wait_for_commit::wait_for_commit()
-  : subsequent_commits_list(0), next_subsequent_commit(0), waitee(0),
-    opaque_pointer(0),
-    waiting_for_commit(false), wakeup_error(0),
-    wakeup_subsequent_commits_running(false)
 {
   mysql_mutex_init(key_LOCK_wait_commit, &LOCK_wait_commit, MY_MUTEX_INIT_FAST);
   mysql_cond_init(key_COND_wait_commit, &COND_wait_commit, 0);
+  reinit();
 }
 
 
@@ -6173,7 +6182,7 @@ wait_for_commit::wakeup(int wakeup_error)
 
   */
   mysql_mutex_lock(&LOCK_wait_commit);
-  waiting_for_commit= false;
+  waitee= NULL;
   this->wakeup_error= wakeup_error;
   /*
     Note that it is critical that the mysql_cond_signal() here is done while
@@ -6205,9 +6214,8 @@ wait_for_commit::wakeup(int wakeup_error)
 void
 wait_for_commit::register_wait_for_prior_commit(wait_for_commit *waitee)
 {
-  waiting_for_commit= true;
-  wakeup_error= 0;
   DBUG_ASSERT(!this->waitee /* No prior registration allowed */);
+  wakeup_error= 0;
   this->waitee= waitee;
 
   mysql_mutex_lock(&waitee->LOCK_wait_commit);
@@ -6217,7 +6225,7 @@ wait_for_commit::register_wait_for_prior_commit(wait_for_commit *waitee)
     see comments on wakeup_subsequent_commits2() for details.
   */
   if (waitee->wakeup_subsequent_commits_running)
-    waiting_for_commit= false;
+    this->waitee= NULL;
   else
   {
     /*
@@ -6247,9 +6255,9 @@ wait_for_commit::wait_for_prior_commit2(THD *thd)
   thd->ENTER_COND(&COND_wait_commit, &LOCK_wait_commit,
                   &stage_waiting_for_prior_transaction_to_commit,
                   &old_stage);
-  while (waiting_for_commit && !thd->check_killed())
+  while ((loc_waitee= this->waitee) && !thd->check_killed())
     mysql_cond_wait(&COND_wait_commit, &LOCK_wait_commit);
-  if (!waiting_for_commit)
+  if (!loc_waitee)
   {
     if (wakeup_error)
       my_error(ER_PRIOR_COMMIT_FAILED, MYF(0));
@@ -6262,7 +6270,6 @@ wait_for_commit::wait_for_prior_commit2(THD *thd)
     waiter as to whether we succeed or fail (eg. we may roll back but waitee
     might attempt to commit both us and any subsequent commits waiting for us).
   */
-  loc_waitee= this->waitee;
   mysql_mutex_lock(&loc_waitee->LOCK_wait_commit);
   if (loc_waitee->wakeup_subsequent_commits_running)
   {
@@ -6271,21 +6278,29 @@ wait_for_commit::wait_for_prior_commit2(THD *thd)
     do
     {
       mysql_cond_wait(&COND_wait_commit, &LOCK_wait_commit);
-    } while (waiting_for_commit);
+    } while (this->waitee);
+    if (wakeup_error)
+      my_error(ER_PRIOR_COMMIT_FAILED, MYF(0));
     goto end;
   }
   remove_from_list(&loc_waitee->subsequent_commits_list);
   mysql_mutex_unlock(&loc_waitee->LOCK_wait_commit);
+  this->waitee= NULL;
 
-  DEBUG_SYNC(thd, "wait_for_prior_commit_killed");
   wakeup_error= thd->killed_errno();
   if (!wakeup_error)
     wakeup_error= ER_QUERY_INTERRUPTED;
   my_message(wakeup_error, ER(wakeup_error), MYF(0));
+  thd->EXIT_COND(&old_stage);
+  /*
+    Must do the DEBUG_SYNC() _after_ exit_cond(), as DEBUG_SYNC is not safe to
+    use within enter_cond/exit_cond.
+  */
+  DEBUG_SYNC(thd, "wait_for_prior_commit_killed");
+  return wakeup_error;
 
 end:
   thd->EXIT_COND(&old_stage);
-  waitee= NULL;
   return wakeup_error;
 }
 
@@ -6368,10 +6383,11 @@ wait_for_commit::wakeup_subsequent_commits2(int wakeup_error)
 void
 wait_for_commit::unregister_wait_for_prior_commit2()
 {
+  wait_for_commit *loc_waitee;
+
   mysql_mutex_lock(&LOCK_wait_commit);
-  if (waiting_for_commit)
+  if ((loc_waitee= this->waitee))
   {
-    wait_for_commit *loc_waitee= this->waitee;
     mysql_mutex_lock(&loc_waitee->LOCK_wait_commit);
     if (loc_waitee->wakeup_subsequent_commits_running)
     {
@@ -6383,7 +6399,7 @@ wait_for_commit::unregister_wait_for_prior_commit2()
         See comments on wakeup_subsequent_commits2() for more details.
       */
       mysql_mutex_unlock(&loc_waitee->LOCK_wait_commit);
-      while (waiting_for_commit)
+      while (this->waitee)
         mysql_cond_wait(&COND_wait_commit, &LOCK_wait_commit);
     }
     else
@@ -6391,10 +6407,10 @@ wait_for_commit::unregister_wait_for_prior_commit2()
       /* Remove ourselves from the list in the waitee. */
       remove_from_list(&loc_waitee->subsequent_commits_list);
       mysql_mutex_unlock(&loc_waitee->LOCK_wait_commit);
+      this->waitee= NULL;
     }
   }
   mysql_mutex_unlock(&LOCK_wait_commit);
-  this->waitee= NULL;
 }
 
 

sql/sql_class.h

@@ -1656,8 +1656,8 @@ struct wait_for_commit
 {
   /*
     The LOCK_wait_commit protects the fields subsequent_commits_list and
-    wakeup_subsequent_commits_running (for a waitee), and the flag
-    waiting_for_commit and associated COND_wait_commit (for a waiter).
+    wakeup_subsequent_commits_running (for a waitee), and the pointer
+    waiterr and associated COND_wait_commit (for a waiter).
   */
   mysql_mutex_t LOCK_wait_commit;
   mysql_cond_t COND_wait_commit;
@@ -1665,7 +1665,13 @@ struct wait_for_commit
   wait_for_commit *subsequent_commits_list;
   /* Link field for entries in subsequent_commits_list. */
   wait_for_commit *next_subsequent_commit;
-  /* Our waitee, if we did register_wait_for_prior_commit(), else NULL. */
+  /*
+    Our waitee, if we did register_wait_for_prior_commit(), and were not
+    yet woken up. Else NULL.
+
+    When this is cleared for wakeup, the COND_wait_commit condition is
+    signalled.
+  */
   wait_for_commit *waitee;
   /*
     Generic pointer for use by the transaction coordinator to optimise the
@@ -1676,12 +1682,6 @@ struct wait_for_commit
     used by another transaction coordinator for similar purposes.
   */
   void *opaque_pointer;
-  /*
-    The waiting_for_commit flag is cleared when a waiter has been woken
-    up. The COND_wait_commit condition is signalled when this has been
-    cleared.
-  */
-  bool waiting_for_commit;
   /* The wakeup error code from the waitee. 0 means no error. */
   int wakeup_error;
   /*
@@ -1697,10 +1697,14 @@ struct wait_for_commit
       Quick inline check, to avoid function call and locking in the common case
       where no wakeup is registered, or a registered wait was already signalled.
     */
-    if (waiting_for_commit)
+    if (waitee)
       return wait_for_prior_commit2(thd);
     else
+    {
+      if (wakeup_error)
+        my_error(ER_PRIOR_COMMIT_FAILED, MYF(0));
       return wakeup_error;
+    }
   }
   void wakeup_subsequent_commits(int wakeup_error)
   {
@@ -1721,7 +1725,7 @@ struct wait_for_commit
   }
   void unregister_wait_for_prior_commit()
   {
-    if (waiting_for_commit)
+    if (waitee)
       unregister_wait_for_prior_commit2();
   }
   /*
@@ -1741,7 +1745,7 @@ struct wait_for_commit
       }
       next_ptr_ptr= &cur->next_subsequent_commit;
     }
-    waiting_for_commit= false;
+    waitee= NULL;
   }
 
   void wakeup(int wakeup_error);
@@ -1752,6 +1756,7 @@ struct wait_for_commit
 
   wait_for_commit();
   ~wait_for_commit();
+  void reinit();
 };
 
 

sql/sys_vars.cc

@@ -1768,6 +1768,49 @@ static Sys_var_ulong Sys_slave_parallel_threads(
        ON_UPDATE(fix_slave_parallel_threads));
 
 
+static bool
+check_slave_domain_parallel_threads(sys_var *self, THD *thd, set_var *var)
+{
+  bool running;
+
+  mysql_mutex_lock(&LOCK_active_mi);
+  running= master_info_index->give_error_if_slave_running();
+  mysql_mutex_unlock(&LOCK_active_mi);
+  if (running)
+    return true;
+
+  return false;
+}
+
+static bool
+fix_slave_domain_parallel_threads(sys_var *self, THD *thd, enum_var_type type)
+{
+  bool running;
+
+  mysql_mutex_unlock(&LOCK_global_system_variables);
+  mysql_mutex_lock(&LOCK_active_mi);
+  running= master_info_index->give_error_if_slave_running();
+  mysql_mutex_unlock(&LOCK_active_mi);
+  mysql_mutex_lock(&LOCK_global_system_variables);
+
+  return running ? true : false;
+}
+
+
+static Sys_var_ulong Sys_slave_domain_parallel_threads(
+       "slave_domain_parallel_threads",
+       "Maximum number of parallel threads to use on slave for events in a "
+       "single replication domain. When using multiple domains, this can be "
+       "used to limit a single domain from grabbing all threads and thus "
+       "stalling other domains. The default of 0 means to allow a domain to "
+       "grab as many threads as it wants, up to the value of "
+       "slave_parallel_threads.",
+       GLOBAL_VAR(opt_slave_domain_parallel_threads), CMD_LINE(REQUIRED_ARG),
+       VALID_RANGE(0,16383), DEFAULT(0), BLOCK_SIZE(1), NO_MUTEX_GUARD,
+       NOT_IN_BINLOG, ON_CHECK(check_slave_domain_parallel_threads),
+       ON_UPDATE(fix_slave_domain_parallel_threads));
+
+
 static Sys_var_ulong Sys_slave_parallel_max_queued(
        "slave_parallel_max_queued",
        "Limit on how much memory SQL threads should use per parallel "