Bharath Vissapragada created HBASE-25984:
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Summary: FSHLog WAL lockup with sync future reuse [RS deadlock]
Key: HBASE-25984
URL: https://issues.apache.org/jira/browse/HBASE-25984
Project: HBase
Issue Type: Bug
Components: regionserver, wal
Affects Versions: 3.0.0-alpha-1, 1.7.0, 2.5.0, 2.4.5
Reporter: Bharath Vissapragada
Assignee: Bharath Vissapragada
We use FSHLog as the WAL implementation (branch-1 based) and under heavy load
we noticed the WAL system gets locked up due to a subtle bug involving racy
code with sync future reuse. This bug applies to all FSHLog implementations
across branches.
Symptoms:
On heavily loaded clusters with large write load we noticed that the region
servers are hanging abruptly with filled up handler queues and stuck MVCC
indicating appends/syncs not making any progress.
{noformat}
WARN [8,queue=9,port=60020] regionserver.MultiVersionConcurrencyControl -
STUCK for : 296000 millis. MultiVersionConcurrencyControl{readPoint=172383686,
writePoint=172383690, regionName=1ce4003ab60120057734ffe367667dca}
WARN [6,queue=2,port=60020] regionserver.MultiVersionConcurrencyControl -
STUCK for : 296000 millis. MultiVersionConcurrencyControl{readPoint=171504376,
writePoint=171504381, regionName=7c441d7243f9f504194dae6bf2622631}
{noformat}
All the handlers are stuck waiting for the sync futures and timing out.
{noformat}
java.lang.Object.wait(Native Method)
org.apache.hadoop.hbase.regionserver.wal.SyncFuture.get(SyncFuture.java:183)
org.apache.hadoop.hbase.regionserver.wal.FSHLog.blockOnSync(FSHLog.java:1509)
.....
{noformat}
Log rolling is stuck because it was unable to attain a safe point
{noformat}
java.util.concurrent.CountDownLatch.await(CountDownLatch.java:277)
org.apache.hadoop.hbase.regionserver.wal.FSHLog$SafePointZigZagLatch.waitSafePoint(FSHLog.java:1799)
org.apache.hadoop.hbase.regionserver.wal.FSHLog.replaceWriter(FSHLog.java:900)
{noformat}
and the Ring buffer consumer thinks that there are some outstanding syncs that
need to finish..
{noformat}
org.apache.hadoop.hbase.regionserver.wal.FSHLog$RingBufferEventHandler.attainSafePoint(FSHLog.java:2031)
org.apache.hadoop.hbase.regionserver.wal.FSHLog$RingBufferEventHandler.onEvent(FSHLog.java:1999)
org.apache.hadoop.hbase.regionserver.wal.FSHLog$RingBufferEventHandler.onEvent(FSHLog.java:1857)
{noformat}
On the other hand, SyncRunner threads are idle and just waiting for work
implying that there are no pending SyncFutures that need to be run
{noformat}
sun.misc.Unsafe.park(Native Method)
java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
org.apache.hadoop.hbase.regionserver.wal.FSHLog$SyncRunner.run(FSHLog.java:1297)
java.lang.Thread.run(Thread.java:748)
{noformat}
Overall the WAL system is dead locked and could make no progress until it was
aborted. I got to the bottom of this issue and have a patch that can fix it
(more details in the comments due to word limit in the description).
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