tldr: This is a bug in EMR 5.6. Upgrading to EMR 5.8 or above can solve the issue.
User reported that he sees 2000+ s3n-worker threads after job finished. He has to restart the hive-server2 service everyday to mitigate the issue.
The threads are repeating from s3n-worker-0 to s3n-worker-19. In another word, there are 100 * 20 s3n-worker threads.
"s3n-worker-1" #52 daemon prio=5 os_prio=0 tid=0x00007f5ac5462000 nid=0x109b waiting on condition [0x00007f5aca23f000]
"s3n-worker-0" #51 daemon prio=5 os_prio=0 tid=0x00007f5ac5480000 nid=0x109a waiting on condition [0x00007f5aca641000]