The performance of "NonPeriodicTasks" pools defined in class ScheduledExecutors is low

We use cassandra as backend storage for Janusgraph. when we loading huge data (~2 billion vertex, ~10 billion edges), we met some problems.

At first, we use STCS as compaction strategy , but met below exception.  we checked the value of  "max memory lock" is unlimited and "file map count" is 1 million, these values should enough for loading data. last we found this problem is caused by the virtual memory are all cosumed by cassandra.  So not additional virtual memory can be used by compaction task , and below exception is thrown out.   

ERROR [CompactionExecutor:267] 2018-08-09 02:28:40,952 JVMStabilityInspector.javv
a:74 - OutOfMemory error letting the JVM handle the error:
java.lang.OutOfMemoryError: Map failed

So, we change compaction strategy to LCS, this change seems can resolve the virtual memory problem. But we found another problem : Many sstables which has been compacted are still retained on disk,  these old sstables consume so many disk space, it's causing no enough disk for saving real data. and we found that many files like "mc_txn_compaction_xxx.log" are created under the data directory. 

After some times' investigaton, found this problem is caused by "NonPeriodicTasks" thread pools.  this pools is always using only one thread for processing clean task after compaction. this thread pool is instanced with class DebuggableScheduledThreadPoolExecutor,

and DebuggableScheduledThreadPoolExecutor is inherit from class  ScheduledThreadPoolExecutor.

By reading the code of class DebuggableScheduledThreadPoolExecutor,  found DebuggableScheduledThreadPoolExecutor is using an unbound task queue, and core pool size is 1. I think it should wrong using unbound queue.  If we using unbound queue, the thread pool wouldn't  increasing thread even there're many tasks are blocked in queue, because unbound queue never would be full.  I think here should use bound queue, so when clean task is heavily, more threads would created for processing them. 

public DebuggableScheduledThreadPoolExecutor(int corePoolSize, String threadPoolName, int priority)
Unknown macro: { super(corePoolSize, new NamedThreadFactory(threadPoolName, priority)); setRejectedExecutionHandler(rejectedExecutionHandler); }
 

public ScheduledThreadPoolExecutor(int corePoolSize,
ThreadFactory threadFactory)
Unknown macro: { super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS, new DelayedWorkQueue(), threadFactory); }

 Below is the case about clean task after compaction.  there nearly 3 hours delay for removing file "mc-56525". 

 

TRACE [CompactionExecutor:81] 2018-08-16 21:22:29,664 LifecycleTransaction.java:363 - Staging for obsolescence BigTableReader(path='/sdb/data/test_2/edgestore-365b0b70a05911e8806001ebe60a5ce7/mc-56525-big-Data.db')
..........
TRACE [CompactionExecutor:81] 2018-08-16 21:22:41,162 Tracker.java:165 - removing /sdb/data/test_2/edgestore-365b0b70a05911e8806001ebe60a5ce7/mc-56525-big from list of files tracked for test_2.edgestore
............
TRACE [NonPeriodicTasks:1] 2018-08-17 00:28:47,179 SSTableReader.java:2175 - Async instance tidier for /sdb/data/test_2/edgestore-365b0b70a05911e8806001ebe60a5ce7/mc-56525-big, before barrier
TRACE [NonPeriodicTasks:1] 2018-08-17 00:28:47,180 SSTableReader.java:2181 - Async instance tidier for /sdb/data/test_2/edgestore-365b0b70a05911e8806001ebe60a5ce7/mc-56525-big, after barrier
TRACE [NonPeriodicTasks:1] 2018-08-17 00:28:47,182 SSTableReader.java:2196 - Async instance tidier for /sdb/data/test_2/edgestore-365b0b70a05911e8806001ebe60a5ce7/mc-56525-big, completed