MDEV-4506: Parallel replication. .result file updates + a few comment updates.

mysql-test/suite/innodb/r/group_commit_binlog_pos.result

@@ -31,6 +31,6 @@ a
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-InnoDB: Last MySQL binlog file position 0 922, file name ./master-bin.000001
+InnoDB: Last MySQL binlog file position 0 926, file name ./master-bin.000001
 SET DEBUG_SYNC= 'RESET';
 DROP TABLE t1;

mysql-test/suite/innodb/r/group_commit_binlog_pos_no_optimize_thread.result

@@ -32,6 +32,6 @@ a
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-InnoDB: Last MySQL binlog file position 0 922, file name ./master-bin.000001
+InnoDB: Last MySQL binlog file position 0 926, file name ./master-bin.000001
 SET DEBUG_SYNC= 'RESET';
 DROP TABLE t1;

sql/rpl_parallel.cc

@@ -647,11 +647,12 @@ rpl_parallel::do_event(rpl_group_info *serial_rgi, Log_event *ev)
         still executing the first ones, to be able to start executing a large
         event group without having to wait for the end to be fetched from the
         master. And we continue to queue up more events after the first group,
-        avoiding the overhead of worker threads constantly entering and
-        leaving the worker thread free list.
+        so that we can continue to process subsequent parts of the relay log in
+        parallel without having to wait for previous long-running events to
+        complete.
 
         But if the worker thread is idle at any point, it may return to the
-        idle list or be servicing a different request. So check this, and
+        idle list or start servicing a different request. So check this, and
         allocate a new thread if the old one is no longer processing for us.
       */
       cur_thread= e->rpl_thread;

sql/rpl_parallel.h

@@ -57,6 +57,10 @@ struct rpl_parallel_entry {
   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.
+  */
   uint64 current_sub_id;
   rpl_group_info *current_group_info;
   /*