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https://issues.apache.org/jira/browse/HBASE-9087?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13725531#comment-13725531
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Lars Hofhansl commented on HBASE-9087:
--------------------------------------
That's why you make sure to have at least as many handlers as CPU threads, and
few more to handle io waits.
Having way more threads than CPU threads and spindles is counter productive and
you're better off queuing request.
> Handlers being blocked during reads
> -----------------------------------
>
> Key: HBASE-9087
> URL: https://issues.apache.org/jira/browse/HBASE-9087
> Project: HBase
> Issue Type: Bug
> Components: Performance
> Affects Versions: 0.94.7, 0.95.1
> Reporter: Pablo Medina
> Assignee: Elliott Clark
> Priority: Critical
> Fix For: 0.98.0, 0.95.2, 0.94.11
>
> Attachments: HBASE-9087-0.patch, HBASE-9087-1.patch
>
>
> I'm having a lot of handlers (90 - 300 aprox) being blocked when reading
> rows. They are blocked during changedReaderObserver registration.
> Lars Hofhansl suggests to change the implementation of changedReaderObserver
> from CopyOnWriteList to ConcurrentHashMap.
> Here is a stack trace:
> "IPC Server handler 99 on 60020" daemon prio=10 tid=0x0000000041c84000
> nid=0x2244 waiting on condition [0x00007ff51fefd000]
> java.lang.Thread.State: WAITING (parking)
> at sun.misc.Unsafe.park(Native Method)
> - parking to wait for <0x00000000c5c13ae8> (a
> java.util.concurrent.locks.ReentrantLock$NonfairSync)
> at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)
> at
> java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:811)
> at
> java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:842)
> at
> java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1178)
> at
> java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:186)
> at
> java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
> at
> java.util.concurrent.CopyOnWriteArrayList.addIfAbsent(CopyOnWriteArrayList.java:553)
> at
> java.util.concurrent.CopyOnWriteArraySet.add(CopyOnWriteArraySet.java:221)
> at
> org.apache.hadoop.hbase.regionserver.Store.addChangedReaderObserver(Store.java:1085)
> at
> org.apache.hadoop.hbase.regionserver.StoreScanner.<init>(StoreScanner.java:138)
> at
> org.apache.hadoop.hbase.regionserver.Store.getScanner(Store.java:2077)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.<init>(HRegion.java:3755)
> at
> org.apache.hadoop.hbase.regionserver.HRegion.instantiateRegionScanner(HRegion.java:1804)
> at
> org.apache.hadoop.hbase.regionserver.HRegion.getScanner(HRegion.java:1796)
> at
> org.apache.hadoop.hbase.regionserver.HRegion.getScanner(HRegion.java:1771)
> at org.apache.hadoop.hbase.regionserver.HRegion.get(HRegion.java:4776)
> at org.apache.hadoop.hbase.regionserver.HRegion.get(HRegion.java:4750)
> at
> org.apache.hadoop.hbase.regionserver.HRegionServer.get(HRegionServer.java:2152)
> at
> org.apache.hadoop.hbase.regionserver.HRegionServer.multi(HRegionServer.java:3700)
> at sun.reflect.GeneratedMethodAccessor26.invoke(Unknown Source)
> at
> sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
> at java.lang.reflect.Method.invoke(Method.java:597)
> at
> org.apache.hadoop.hbase.ipc.WritableRpcEngine$Server.call(WritableRpcEngine.java:320)
> at
> org.apache.hadoop.hbase.ipc.HBaseServer$Handler.run(HBaseServer.java:1426)
>
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