Slave sql thread blocked by Update_rows_log_event::find_row(-1)

There are more than 32 thread running on the instance,lead to many slow log,but user's connections are still running,but slow than normal.I check engine innodb status found these messages:

Trx #rec lock waits 320708 #table lock waits 0
Trx total rec lock wait time 792 SEC
Trx total table lock wait time 0 SEC
---TRANSACTION 86665517604, ACTIVE (PREPARED) 0 sec
3 lock struct(s), heap size 1184, 1 row lock(s), undo log entries 2
MySQL thread id 878349428, OS thread handle 0x7fb5255fab00, query id 189853061018 Unlocking tables
Trx #rec lock waits 318751 #table lock waits 0
Trx total rec lock wait time 774 SEC
Trx total table lock wait time 0 SEC
---TRANSACTION 86665517602, ACTIVE (PREPARED) 0 sec
3 lock struct(s), heap size 1184, 1 row lock(s), undo log entries 2
–
878349423 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349424 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349425 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349426 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349427 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349428 system user NULL Connect 0 Update_rows_log_event::find_row(-1) NULL 0.000
878349429 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349430 system user NULL Connect 0 Waiting for prior transaction to start commit before starting next transaction NULL 0.000
878349431 system user NULL Connect 799 Slave has read all relay log; waiting for the slave I/O thread to update it NULL 0.000
889402627 repl IP1:13232 NULL Binlog Dump 2062590 Master has sent all binlog to slave; waiting for binlog to be updated NULL 0.000
889832413 repl IP2:59589 NULL Binlog Dump 1782552 Master has sent all binlog to slave; waiting for binlog to be updated NULL 0.000
–
---TRANSACTION 2819, not started
MySQL thread id 1, OS thread handle 0x7fb554d58b00, query id 0 Waiting for background binlog tasks
---TRANSACTION 86665661686, ACTIVE 0 sec starting index read
mysql tables in use 1, locked 1
1 lock struct(s), heap size 360, 0 row lock(s)
MySQL thread id 878349428, OS thread handle 0x7fb5255fab00, query id 189853425690 Update_rows_log_event::find_row(-1)
Trx #rec lock waits 318754 #table lock waits 0
Trx total rec lock wait time 774 SEC
Trx total table lock wait time 0 SEC
--------
ROW OPERATIONS
--------------
32 queries inside InnoDB, 134 queries in queue
33 read views open inside InnoDB
2 RW transactions active inside InnoDB
0 RO transactions active inside InnoDB
2 out of 1000 descriptors used

I killed the thread 878349428,and user's queries become normal.
and then I found slave io thread is not running,so I started it.then everything was ok.