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https://issues.apache.org/jira/browse/HBASE-17434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15814335#comment-15814335
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Duo Zhang commented on HBASE-17434:
-----------------------------------
{quote}
Volatile - I follow the oracle documentation in
https://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html which
says reads and writes of reference variable are always atomic, while long and
double are atomic if declared volatile. Therefor, version which is long needs
to be volatile, but readOnlyCopy is ok without a volatile.
{quote}
The readOnlyCopy is just a pointer actually, which means it is a long... And
even if java can guarantee atomic access for a long without volatile, it does
not mean you can just avoid the volatile here. volatile has other side effect
as it ensures happens-before order. See this:
https://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#volatile
And one example is CopyOnWriteArrayList, the array field is declared as
volatile. This is a class in jdk and written by Doug Lea, the king of
concurrent programming in java.
Thanks.
> 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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