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From "Yu Li (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17747) Support both weak and soft object pool
Date Tue, 14 Mar 2017 04:23:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17747?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15923548#comment-15923548
] 

Yu Li commented on HBASE-17747:
-------------------------------

Thanks [~stack] and [~Apache9] for your concerns, such concerns make our project better (smile).

Allow me to quote some words from [javadoc|https://docs.oracle.com/javase/7/docs/api/java/lang/ref/SoftReference.html]:
{noformat}
Soft reference objects, which are cleared at the discretion of the garbage collector in response
to memory demand.
Soft references are most often used to implement memory-sensitive caches.
...
Direct instances of this class may be used to implement simple caches; this class or derived
subclasses may also be used
in larger data structures to implement more sophisticated caches. As long as the referent
of a soft reference is strongly reachable,
that is, is actually in use, the soft reference will not be cleared. Thus a sophisticated
cache can, for example, prevent its most
recently used entries from being discarded by keeping strong referents to those entries, leaving
the remaining entries to be
discarded at the discretion of the garbage collector.
{noformat}

> Support both weak and soft object pool
> --------------------------------------
>
>                 Key: HBASE-17747
>                 URL: https://issues.apache.org/jira/browse/HBASE-17747
>             Project: HBase
>          Issue Type: Improvement
>    Affects Versions: 2.0
>            Reporter: Yu Li
>            Assignee: Yu Li
>             Fix For: 2.0
>
>         Attachments: HBASE-17747.patch, HBASE-17747.v2.patch, HBASE-17747.v3.patch
>
>
> During YCSB testing on embedded mode after HBASE-17744, we found that under high read
load GC is quite severe even with offheap L2 cache. After some investigation, we found it's
caused by using weak reference in {{IdReadWriteLock}}. In embedded mode the read is so quick
that the lock might already get promoted to the old generation when the weak reference is
cleared, which causes dirty card table (old reference get removed and new lock object set
into {{referenceCache}}, see {{WeakObjectPool#get}}) thus slowing YGC. In distributed mode
there'll also be more lock object created with weak reference than soft reference that slowing
down the processing.
> So we proposed to use soft reference for this {{IdReadWriteLock}} used in cache, which
won't get cleared until JVM memory is not enough, and could resolve the issue mentioned above.
What's more, we propose to extend the {{WeakObjectPool}} to be more generate to support both
weak and soft reference.
> Note that the GC issue only emerges under embedded mode with DirectOperator, in which
case all costs on the wire is removed thus produces extremely high concurrency.



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