The relay-log is not flushed after the slave-relay-log.999999 showed

I am using MariaDB 10.0.14 at CentOS 6.5.
And, as I mentioned at the title of this question, the relay-log is not
flushed after the slave-relay-log.999999 showed when using
"Salve_parallel_threads:10" setting. like showed blow.

• binlog_format: ROW
• Slave_parallel_threads:10

Everything are working fine except the "slave-relay-log.******" files
continue to exist at the disk which will finally cause the disk full.
If I change the value of Slave_parallel_threads setting from 10 to 0,
the log will be flushed. Howevery "PK duplicate warning" error logs come
next.

Is there any setting should be used with Slave_parallel_threads setting?
Any help will be great.

Best regareds,