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From "Josh Rosen (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SPARK-6209) ExecutorClassLoader can leak connections after failing to load classes from the REPL class server
Date Fri, 06 Mar 2015 22:57:38 GMT
Josh Rosen created SPARK-6209:
---------------------------------

             Summary: ExecutorClassLoader can leak connections after failing to load classes
from the REPL class server
                 Key: SPARK-6209
                 URL: https://issues.apache.org/jira/browse/SPARK-6209
             Project: Spark
          Issue Type: Bug
          Components: Spark Core
    Affects Versions: 1.2.1, 1.0.0, 1.0.3, 1.3.0, 1.1.2, 1.4.0
            Reporter: Josh Rosen
            Assignee: Josh Rosen
            Priority: Critical


ExecutorClassLoader does not ensure proper cleanup of network connections that it opens. 
If it fails to load a class, it may leak partially-consumed InputStreams that are connected
to the REPL's HTTP class server, causing that server to exhaust its thread pool, which can
cause the entire job to hang.

Here is a simple reproduction:

With

{code}
./bin/spark-shell --master local-cluster[8,8,512] 
{code}

run the following command:

{code}
sc.parallelize(1 to 1000, 1000).map { x =>
  try {
      Class.forName("some.class.that.does.not.Exist")
  } catch {
      case e: Exception => // do nothing
  }
  x
}.count()
{code}

This job will run 253 tasks, then will completely freeze without any errors or failed tasks.

It looks like the driver has 253 threads blocked in socketRead0() calls:

{code}
[joshrosen ~]$ jstack 16765 | grep socketRead0 | wc
     253     759   14674
{code}

e.g.

{code}
"qtp1287429402-13" daemon prio=5 tid=0x00007f868a1c0000 nid=0x5b03 runnable [0x00000001159bd000]
   java.lang.Thread.State: RUNNABLE
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.read(SocketInputStream.java:152)
    at java.net.SocketInputStream.read(SocketInputStream.java:122)
    at org.eclipse.jetty.io.ByteArrayBuffer.readFrom(ByteArrayBuffer.java:391)
    at org.eclipse.jetty.io.bio.StreamEndPoint.fill(StreamEndPoint.java:141)
    at org.eclipse.jetty.server.bio.SocketConnector$ConnectorEndPoint.fill(SocketConnector.java:227)
    at org.eclipse.jetty.http.HttpParser.fill(HttpParser.java:1044)
    at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:280)
    at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
    at org.eclipse.jetty.server.BlockingHttpConnection.handle(BlockingHttpConnection.java:72)
    at org.eclipse.jetty.server.bio.SocketConnector$ConnectorEndPoint.run(SocketConnector.java:264)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
    at java.lang.Thread.run(Thread.java:745) 
{code}

Jstack on the executors shows blocking in loadClass / findClass, where a single thread is
RUNNABLE and waiting to hear back from the driver and other executor threads are BLOCKED on
object monitor synchronization at Class.forName0().

Remotely triggering a GC on a hanging executor allows the job to progress and complete more
tasks before hanging again.  If I repeatedly trigger GC on all of the executors, then the
job runs to completion:

{code}
jps | grep CoarseGra | cut -d ' ' -f 1 | xargs -I {} -n 1 -P100 jcmd {} GC.run
{code}

The culprit is a {{catch}} block that ignores all exceptions and performs no cleanup: https://github.com/apache/spark/blob/v1.2.0/repl/src/main/scala/org/apache/spark/repl/ExecutorClassLoader.scala#L94

This bug has been present since Spark 1.0.0, but I suspect that we haven't seen it before
because it's pretty hard to reproduce. Triggering this error requires a job with tasks that
trigger ClassNotFoundExceptions yet are still able to run to completion.  It also requires
that executors are able to leak enough open connections to exhaust the class server's Jetty
thread pool limit, which requires that there are a large number of tasks (253+) and either
a large number of executors or a very low amount of GC pressure on those executors (since
GC will cause the leaked connections to be closed).

The fix here is pretty simple: add proper resource cleanup to this class.




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