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From "Simon Zhou (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-13555) Thread leak during repair
Date Thu, 25 May 2017 21:53:04 GMT
Simon Zhou created CASSANDRA-13555:

             Summary: Thread leak during repair
                 Key: CASSANDRA-13555
             Project: Cassandra
          Issue Type: Bug
            Reporter: Simon Zhou
            Assignee: Simon Zhou

The symptom is similar to what happened in [CASSANDRA-13204 |]
that the thread waiting forever doing nothing. This one happened during "nodetool repair -pr
-seq -j 1" in production but I can easily simulate the problem with just "nodetool repair"
in dev environment (CCM). I'm trying to explain what happened with 3.0.13 code base.

1. One node is down while doing repair. This is the error I saw in production:

ERROR [GossipTasks:1] 2017-05-19 15:00:10,545 - [repair #bc9a3cd1-3ca3-11e7-a44a-e30923ac9336]
session completed with the following error Endpoint / died
    at ~[apache-cassandra-3.0.11.jar:3.0.11]
    at org.apache.cassandra.gms.FailureDetector.interpret( [apache-cassandra-3.0.11.jar:3.0.11]
    at org.apache.cassandra.gms.Gossiper.doStatusCheck( [apache-cassandra-3.0.11.jar:3.0.11]
    at org.apache.cassandra.gms.Gossiper.access$800( [apache-cassandra-3.0.11.jar:3.0.11]
    at org.apache.cassandra.gms.Gossiper$ [apache-cassandra-3.0.11.jar:3.0.11]
    at org.apache.cassandra.concurrent.DebuggableScheduledThreadPoolExecutor$
    at java.util.concurrent.Executors$ [na:1.8.0_121]
    at java.util.concurrent.FutureTask.runAndReset( [na:1.8.0_121]
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(
    at java.util.concurrent.ScheduledThreadPoolExecutor$
    at java.util.concurrent.ThreadPoolExecutor.runWorker( [na:1.8.0_121]
    at java.util.concurrent.ThreadPoolExecutor$ [na:1.8.0_121]
    at org.apache.cassandra.concurrent.NamedThreadFactory.lambda$threadLocalDeallocator$0(
    at ~[na:1.8.0_121]

2. At this moment the repair coordinator hasn't received the response (MerkleTrees) for the
node that was marked down. This means, RepairJob#run will never return because it waits for
validations to finish:

        // Wait for validation to complete

Be noted that all RepairJob's (as Runnable) run on a shared executor created in RepairRunnable#runMayThrow,
while all snapshot, validation and sync'ing happen on a per-RepairSession "taskExecutor".
The RepairJob#run will only return when it receives MerkleTrees (or null) from all endpoints
for a given column family and token range.

As evidence of the thread leak, below is from the thread dump. I can also get the same stack
trace when simulating the same issue in dev environment.

"Repair#129:56" #406373 daemon prio=5 os_prio=0 tid=0x00007fc495028400 nid=0x1a77d waiting
on condition [0x00007fc021530000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00000002d7c00198> (a$Sync)
    at java.util.concurrent.locks.LockSupport.park(
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(
    at java.util.concurrent.ThreadPoolExecutor.runWorker(
    at java.util.concurrent.ThreadPoolExecutor$
    at org.apache.cassandra.concurrent.NamedThreadFactory.lambda$threadLocalDeallocator$0(
    at org.apache.cassandra.concurrent.NamedThreadFactory$$Lambda$4/

   Locked ownable synchronizers:
    - <0x00000002d7c00230> (a java.util.concurrent.ThreadPoolExecutor$Worker)

So here are two things:
1. For the thread leak itself, either we do something like below in RepairSession#terminate,
or we use timed wait at the end of RepairJob#run.
        for (ValidationTask validationTask : validating.values()) {

2. Another question is, instead of waiting for synchronization (SyncTask) to finish, why we
just wait for validation? Is it because we want to speed things up and anyway we have throttling
on streaming?

[~yukim] I'd love to get your comment. I'll check if this issue exists in other versions.

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