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https://issues.apache.org/jira/browse/HDFS-15150?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17030188#comment-17030188
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Wei-Chiu Chuang commented on HDFS-15150:
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{code:java}
this.datasetRWLock = new InstrumentedReadWriteLock(true,
{code}
This means the dataset lock will be a fair RW lock. I wonder if we should make
it configurable. Looking at the ReentrantReadWriteLock usage in namenode,
(HDFS-5241) unfair lock outperforms fair lock.
Unrelated, but the following existing code looks suspicious:
{code}
volumes.waitVolumeRemoved(5000, datasetWriteLockCondition);
{code}
This condition is never notified by another thread. Wonder how it worked
before. I need to dust off the Java concurrency.
> Introduce read write lock to Datanode
> -------------------------------------
>
> Key: HDFS-15150
> URL: https://issues.apache.org/jira/browse/HDFS-15150
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 3.3.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: HDFS-15150.001.patch, HDFS-15150.002.patch
>
>
> HDFS-9668 pointed out the issues around the DN lock being a point of
> contention some time ago, but that Jira went in a direction of creating a new
> FSDataset implementation which is very risky, and activity on the Jira has
> stalled for a few years now. Edit: Looks like HDFS-9668 eventually went in a
> similar direction to what I was thinking, so I will review that Jira in more
> detail to see if this one is necessary.
> I feel there could be significant gains by moving to a ReentrantReadWrite
> lock within the DN. The current implementation is simply a ReentrantLock so
> any locker blocks all others.
> Once place I think a read lock would benefit us significantly, is when the DN
> is serving a lot of small blocks and there are jobs which perform a lot of
> reads. The start of reading any blocks right now takes the lock, but if we
> moved this to a read lock, many reads could do this at the same time.
> The first conservative step, would be to change the current lock and then
> make all accesses to it obtain the write lock. That way, we should keep the
> current behaviour and then we can selectively move some lock accesses to the
> readlock in separate Jiras.
> I would appreciate any thoughts on this, and also if anyone has attempted it
> before and found any blockers.
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