测试步骤：
从库停止复制：stop slave;
主库创建大表400万条记录。
开启从库复制：start slave;
监测从库error log持续输出：
2018-12-06T10:40:52.616289+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2431 times hitting slave_pending_jobs_size_max; current event size = 8207.
2018-12-06T10:40:52.647618+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2441 times hitting slave_pending_jobs_size_max; current event size = 8207.
2018-12-06T10:40:52.679589+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2451 times hitting slave_pending_jobs_size_max; current event size = 8207.
2018-12-06T10:40:52.711510+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2461 times hitting slave_pending_jobs_size_max; current event size = 8207.
2018-12-06T10:40:52.750250+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2471 times hitting slave_pending_jobs_size_max; current event size = 8207.
2018-12-06T10:40:52.785731+08:00 4 [Note] Multi-threaded slave: Coordinator has waited 2481 times hitting slave_pending_jobs_size_max; current event size = 8207.
第一种
Last_Error: Cannot schedule event Rows_query, relay-log name ./db-s18-relay-bin.000448, position 419156572 to Worker thread because its size 18483519 exceeds 16777216 of slave_pending_jobs_size_max.
第二种
[Note] Multi-threaded slave: Coordinator has waited 701 times hitting slave_pending_jobs_size_max; current event size = 8167.
BUG地址：https://bugs.mysql.com/bug.php?id=68462
以上两种报错，初步判断问题可能在 slave_pending_jobs_size_max 的大小上，此值，官方默认是 16M，此值可以动态调整
在多线程复制时，在队列中Pending的事件所占用的最大内存，默认为16M，如果内存富余，或者延迟较大时，可以适当调大;注意这个值要比主库的max_allowed_packet大
1.- 如果event大小已经超过了等待任务大小的上限（配置slave-pending-jobs-size-max )，就报event太大的错，然后返回；
2.- 如果event大小+已经在等待的任务大小超过了slave-pending-jobs-size-max，就等待，至到等待队列变小；
3.- 如果当前的worker的队列满的话，也等待。
---------------------
| Variable_name | Value |
+-----------------------------+----------+
| slave_pending_jobs_size_max | 16777216 |
+-----------------------------+----------+
Query OK, 0 rows affected (0.02 sec)
+-----------------------------+-----------+
| Variable_name | Value |
+-----------------------------+-----------+
| slave_pending_jobs_size_max | 134217728 |
+-----------------------------+-----------+
1 row in set (0.00 sec)