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From mattgordon <matt.gor...@crowdflower.com>
Subject Connection closed exceptions with slow fsync and CancelledKeyExceptions
Date Thu, 08 Nov 2012 01:48:14 GMT
We have been trying to understand why our ZooKeeper cluster will occasionally
have a wave of connection closed exceptions. We have switched to
-XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode for garbage collection with
no noticeable improvements.

The symptoms are:

(1) All nodes show messages like "fsync-ing the write ahead log in
SyncThread:0 took 6309ms which will adversely effect operation latency. See
the ZooKeeper troubleshooting guide" with times typically around 5 seconds.
At least once, this fsync appeared in the leaders log immediately before a
wave of:

ERROR [CommitProcessor:0:NIOServerCnxn@445] - Unexpected Exception: 
	at sun.nio.ch.SelectionKeyImpl.ensureValid(SelectionKeyImpl.java:73)
	at sun.nio.ch.SelectionKeyImpl.interestOps(SelectionKeyImpl.java:77)

Our clients received ZookeeperConnectionClosed exceptions at this time and
all traffic on the ZooKeeper cluster essentially went to zero for a moment
before resuming normal operation with new connections.

(2) Probably unrelated since I haven't correlated it temporally with the
client errors, but running "sudo strace -r -T -f -p 9574 -e
trace=fsync,fdatasync -o trace.txt" turns up some messages like "10581
0.000246 — SIGSEGV (Segmentation fault) @ 0 (0) —"

ZK Version: 3.3.4
Cluster has 5 nodes running in EC2

Here is a screenshot showing ZooKeeper network traffic going to zero at the
time of the connection closed exceptions: http://i.imgur.com/dfNh0.png

Anyone have ideas on what the cause of these "waves" of
CancelledKeyExceptions could be from?

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