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https://issues.apache.org/jira/browse/HADOOP-14938?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Misha Dmitriev updated HADOOP-14938:
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Attachment: HADOOP-14938.02.patch
Addressed Daniel's comments.
I've added {{volatile}} as you suggested, but note that from my understanding
of the Java Memory Model and experience writing concurrent apps, it is not
necessary here. What happens is that since initially {{updatingResource}} is
null, and all threads see it as null, as expected. Once some thread decides to
call {{putIntoUpdatingResource()}}, it again first reads this field it as null,
then, after entering the {{synchronized}} block, reads its real value - which
may be non-null if some other thread got ahead and already set this field. What
is important is that when a thread exits the {{synchronized}} block, it has the
effect of flushing the cache to main memory, so that writes made by this thread
become visible to other threads. So even if {{updatingResource}} is not
volatile, its new, non-null value will be visible to all threads that will
subsequently read it.
> Configuration.updatingResource map should be initialized lazily
> ---------------------------------------------------------------
>
> Key: HADOOP-14938
> URL: https://issues.apache.org/jira/browse/HADOOP-14938
> Project: Hadoop Common
> Issue Type: Improvement
> Reporter: Misha Dmitriev
> Assignee: Misha Dmitriev
> Attachments: HADOOP-14938.01.patch, HADOOP-14938.02.patch
>
>
> Using jxray (www.jxray.com), I've analyzed a heap dump of YARN RM running in
> a big cluster. The tool uncovered several inefficiencies in the RM memory. It
> turns out that one of the biggest sources of memory waste, responsible for
> almost 1/4 of used memory, is empty ConcurrentHashMap instances in
> org.apache.hadoop.conf.Configuration.updatingResource:
> {code}
> 905,551K (24.0%): java.util.concurrent.ConcurrentHashMap: 22118 / 100% of
> empty 905,551K (24.0%)
> ↖org.apache.hadoop.conf.Configuration.updatingResource
> ↖{j.u.WeakHashMap}.keys
> ↖Java Static org.apache.hadoop.conf.Configuration.REGISTRY
> {code}
> That is, there are 22118 empty ConcurrentHashMaps here, and they collectively
> waste ~905MB of memory. This is caused by eager initialization of these maps.
> To address this problem, we should initialize them lazily.
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