Remove unnecessary global mutex in parallel replication.

The function apply_event_and_update_pos() is called with the
rli->data_lock mutex held. However, there seems to be nothing in the
function actually needing the mutex to be held. Certainly not in the
parallel replication case, where sql_slave_skip_counter is always 0
since the non-zero case is handled by the SQL driver thread.

So this patch makes parallel replication use a variant of
apply_event_and_update_pos() without the need to take the
rli->data_lock mutex. This avoids one contended global mutex for each
event executed, which might improve performance on CPU-bound workloads
somewhat.

Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

Author: knielsen

sql/rpl_parallel.cc

@@ -47,9 +47,7 @@ rpt_handle_event(rpl_parallel_thread::queued_event *qev,
   if (!(ev->is_artificial_event() || ev->is_relay_log_event() ||
         (ev->when == 0)))
     rgi->last_master_timestamp= ev->when + (time_t)ev->exec_time;
-  mysql_mutex_lock(&rli->data_lock);
-  /* Mutex will be released in apply_event_and_update_pos(). */
-  err= apply_event_and_update_pos(ev, thd, rgi, rpt);
+  err= apply_event_and_update_pos_for_parallel(ev, thd, rgi);
 
   thread_safe_increment64(&rli->executed_entries);
   /* ToDo: error handling. */

sql/slave.cc

@@ -3377,39 +3377,17 @@ has_temporary_error(THD *thd)
 }
 
 
-/**
-  Applies the given event and advances the relay log position.
-
-  In essence, this function does:
-
-  @code
-    ev->apply_event(rli);
-    ev->update_pos(rli);
-  @endcode
-
-  But it also does some maintainance, such as skipping events if
-  needed and reporting errors.
-
-  If the @c skip flag is set, then it is tested whether the event
-  should be skipped, by looking at the slave_skip_counter and the
-  server id.  The skip flag should be set when calling this from a
-  replication thread but not set when executing an explicit BINLOG
-  statement.
-
-  @retval 0 OK.
-
-  @retval 1 Error calling ev->apply_event().
+/*
+  First half of apply_event_and_update_pos(), see below.
+  Setup some THD variables for applying the event.
 
-  @retval 2 No error calling ev->apply_event(), but error calling
-  ev->update_pos().
+  Split out so that it can run with rli->data_lock held in non-parallel
+  replication, but without the mutex held in the parallel case.
 */
-int apply_event_and_update_pos(Log_event* ev, THD* thd,
-                               rpl_group_info *rgi,
-                               rpl_parallel_thread *rpt)
+static int
+apply_event_and_update_pos_setup(Log_event* ev, THD* thd, rpl_group_info *rgi)
 {
-  int exec_res= 0;
-  Relay_log_info* rli= rgi->rli;
-  DBUG_ENTER("apply_event_and_update_pos");
+  DBUG_ENTER("apply_event_and_update_pos_setup");
 
   DBUG_PRINT("exec_event",("%s(type_code: %d; server_id: %d)",
                            ev->get_type_str(), ev->get_type_code(),
@@ -3459,13 +3437,23 @@ int apply_event_and_update_pos(Log_event* ev, THD* thd,
     (ev->flags & LOG_EVENT_SKIP_REPLICATION_F ? OPTION_SKIP_REPLICATION : 0);
   ev->thd = thd; // because up to this point, ev->thd == 0
 
-  int reason= ev->shall_skip(rgi);
-  if (reason == Log_event::EVENT_SKIP_COUNT)
-  {
-    DBUG_ASSERT(rli->slave_skip_counter > 0);
-    rli->slave_skip_counter--;
-  }
-  mysql_mutex_unlock(&rli->data_lock);
+  DBUG_RETURN(ev->shall_skip(rgi));
+}
+
+
+/*
+  Second half of apply_event_and_update_pos(), see below.
+
+  Do the actual event apply (or skip), and position update.
+ */
+static int
+apply_event_and_update_pos_apply(Log_event* ev, THD* thd, rpl_group_info *rgi,
+                                 int reason)
+{
+  int exec_res= 0;
+  Relay_log_info* rli= rgi->rli;
+
+  DBUG_ENTER("apply_event_and_update_pos_apply");
   DBUG_EXECUTE_IF("inject_slave_sql_before_apply_event",
     {
       DBUG_ASSERT(!debug_sync_set_action
@@ -3553,6 +3541,72 @@ int apply_event_and_update_pos(Log_event* ev, THD* thd,
 }
 
 
+/**
+  Applies the given event and advances the relay log position.
+
+  In essence, this function does:
+
+  @code
+    ev->apply_event(rli);
+    ev->update_pos(rli);
+  @endcode
+
+  But it also does some maintainance, such as skipping events if
+  needed and reporting errors.
+
+  If the @c skip flag is set, then it is tested whether the event
+  should be skipped, by looking at the slave_skip_counter and the
+  server id.  The skip flag should be set when calling this from a
+  replication thread but not set when executing an explicit BINLOG
+  statement.
+
+  @retval 0 OK.
+
+  @retval 1 Error calling ev->apply_event().
+
+  @retval 2 No error calling ev->apply_event(), but error calling
+  ev->update_pos().
+
+  This function is only used in non-parallel replication, where it is called
+  with rli->data_lock held; this lock is released during this function.
+*/
+int
+apply_event_and_update_pos(Log_event* ev, THD* thd, rpl_group_info *rgi)
+{
+  Relay_log_info* rli= rgi->rli;
+  mysql_mutex_assert_owner(&rli->data_lock);
+  int reason= apply_event_and_update_pos_setup(ev, thd, rgi);
+  if (reason == Log_event::EVENT_SKIP_COUNT)
+  {
+    DBUG_ASSERT(rli->slave_skip_counter > 0);
+    rli->slave_skip_counter--;
+  }
+  mysql_mutex_unlock(&rli->data_lock);
+  return apply_event_and_update_pos_apply(ev, thd, rgi, reason);
+}
+
+
+/*
+  The version of above apply_event_and_update_pos() used in parallel
+  replication. Unlike the non-parallel case, this function is called without
+  rli->data_lock held.
+*/
+int
+apply_event_and_update_pos_for_parallel(Log_event* ev, THD* thd,
+                                        rpl_group_info *rgi)
+{
+  Relay_log_info* rli= rgi->rli;
+  mysql_mutex_assert_not_owner(&rli->data_lock);
+  int reason= apply_event_and_update_pos_setup(ev, thd, rgi);
+  /*
+    In parallel replication, sql_slave_skip_counter is handled in the SQL
+    driver thread, so 23 should never see EVENT_SKIP_COUNT here.
+  */
+  DBUG_ASSERT(reason != Log_event::EVENT_SKIP_COUNT);
+  return apply_event_and_update_pos_apply(ev, thd, rgi, reason);
+}
+
+
 /**
    Keep the relay log transaction state up to date.
 
@@ -3803,7 +3857,7 @@ static int exec_relay_log_event(THD* thd, Relay_log_info* rli,
     serial_rgi->future_event_relay_log_pos= rli->future_event_relay_log_pos;
     serial_rgi->event_relay_log_name= rli->event_relay_log_name;
     serial_rgi->event_relay_log_pos= rli->event_relay_log_pos;
-    exec_res= apply_event_and_update_pos(ev, thd, serial_rgi, NULL);
+    exec_res= apply_event_and_update_pos(ev, thd, serial_rgi);
 
 #ifdef WITH_WSREP
     WSREP_DEBUG("apply_event_and_update_pos() result: %d", exec_res);

sql/slave.h

@@ -243,8 +243,9 @@ void set_slave_thread_default_charset(THD *thd, rpl_group_info *rgi);
 int rotate_relay_log(Master_info* mi);
 int has_temporary_error(THD *thd);
 int apply_event_and_update_pos(Log_event* ev, THD* thd,
-                               struct rpl_group_info *rgi,
-                               rpl_parallel_thread *rpt);
+                               struct rpl_group_info *rgi);
+int apply_event_and_update_pos_for_parallel(Log_event* ev, THD* thd,
+                                            struct rpl_group_info *rgi);
 
 pthread_handler_t handle_slave_io(void *arg);
 void slave_output_error_info(rpl_group_info *rgi, THD *thd);