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https://issues.apache.org/jira/browse/HBASE-17434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15814832#comment-15814832
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Eshcar Hillel commented on HBASE-17434:
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Java does not have pointers. It has primitive variables and reference variable.
(some say this is just semantics, ...)
We do not need to ensure happens-before relationship here. This should be
handled at higher level. Can you think of a scenario where this can be a
problem?
The way volatile ensures happens-before is by using memory fences, and we
already have them in the locks. And since this variable is always updated at
the context of a lock its value is not stall for too long.
Having said that, if this is a blocker for the Jira then I don't mind adding
the volatile.
I value these discussions because it's important to understand why we do things.
So thanks again for all comments.
> New Synchronization Scheme for Compaction Pipeline
> --------------------------------------------------
>
> Key: HBASE-17434
> URL: https://issues.apache.org/jira/browse/HBASE-17434
> Project: HBase
> Issue Type: Bug
> Reporter: Eshcar Hillel
> Assignee: Eshcar Hillel
> Attachments: HBASE-17434-V01.patch, HBASE-17434-V02.patch,
> HBASE-17434-V03.patch, HBASE-17434-V04.patch, HBASE-17434.master.001.patch
>
>
> A new copyOnWrite synchronization scheme is introduced for the compaction
> pipeline.
> The new scheme is better since it removes the lock from getSegments() which
> is invoked in every get and scan operation, and it reduces the number of
> LinkedList objects that are created at runtime, thus can reduce GC (not by
> much, but still...).
> In addition, it fixes the method getTailSize() in compaction pipeline. This
> method creates a MemstoreSize object which comprises the data size and the
> overhead size of the segment and needs to be atomic.
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