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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-1790) review use of synchronized methods in FSNamesystem
Date Sat, 26 Mar 2011 05:05:05 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13011564#comment-13011564

dhruba borthakur commented on HDFS-1790:

The setting of safemode should be done inside the FSNamesystem write lock. The checking of
safemode should be done inside the  FSNamesystem read lock. But can you explain why the SafeMode
object needs its own "synchronization"?

> review use of synchronized methods in FSNamesystem
> --------------------------------------------------
>                 Key: HDFS-1790
>                 URL: https://issues.apache.org/jira/browse/HDFS-1790
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>    Affects Versions: 0.23.0
>            Reporter: Matt Foley
>             Fix For: 0.23.0
> While analyzing SafeMode semantics for HDFS-1726, I noticed the following usage of synchronized
methods in FSNamesystem that raised questions:
> 1. incVolumeFailure() is synchronized on the FSNamesystem instance (monitor lock), for
unclear reasons.  Is this a left-over from the conversion to read/write lock in FSNamesystem?
> 2. isInSafeMode() and isPopulatingReplQueues() are also synchronized on the FSNamesystem
instance (monitor lock), but the SafeModeInfo methods they call (safeMode.isInSafeMode() and
safeMode.isPopulatingReplQueues() are synchronized on the SafeModeInfo instance.  Is synchronizing
on the FSNamesystem instance necessary?  What does it add?  If it is necessary, should it
be using the read/write lock?
> 3. In SafeModeInfo, these methods are synchronized on the SafeModeInfo instance:
> - isOn
> - isPopulatingReplQueues
> - leave
> - initialize
> - canInitializeReplQueues
> - canLeave
> - setBlock
> - incrementSafeBlockCount
> - decrementSafeBlockCount
> - setManual
> but these are not:
> - enter
> - needEnter
> - checkMode
> - isManual
> - isConsistent
> Regarding these:
> - isOn() asserts isConsistent(), but is otherwise an atomic read operation.
> - isPopulatingReplQueues() is an atomic read.  Does it need synchronization?
> - leave() is complex, but shouldn't it be synchronized with enter(), which is also a
write operation?  Yet enter() is unsynchronized.
> - initializeReplQueues() calls blockManager.processMisReplicatedBlocks(), which can take
minutes to run.  By synchronizing on the SafeModeInfo instance, it prevents essentially all
of the other safeMode methods from running for the duration!  Is this desirable or needed?
> - canInitializeReplQueues() is a read-only operation, although it does compare two read
values.  needEnter() compares four read values, and is unsynchronized.  Does either need synchronization?
> - canLeave() is a compound operation, it's good that it is synchronized.
> - checkMode() is a big compound read/write operation.  Doesn't it need synchronization
if the other methods do?
> and so on.

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