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https://issues.apache.org/jira/browse/HDFS-11383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16017902#comment-16017902
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Misha Dmitriev commented on HDFS-11383:
---------------------------------------
Hi [~manojg], in response to your question about String.intern() performance.
Here is the best article on this that I found:
http://java-performance.info/string-intern-in-java-6-7-8/
One takeaway from it is that the internal hashtable used by the JVM for
String.intern() is fixed-size and preallocated. So you essentially pay the
memory price of it upfront whether you use it or not, but then it's free,
unlike Guava. This implementation uses internal JVM pointers and other
optimizations only possible inside the JVM, so it's faster and more economical
than anything that can be written in Java.
> String duplication in org.apache.hadoop.fs.BlockLocation
> --------------------------------------------------------
>
> Key: HDFS-11383
> URL: https://issues.apache.org/jira/browse/HDFS-11383
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Misha Dmitriev
> Assignee: Misha Dmitriev
> Attachments: HDFS-11383.01.patch
>
>
> I am working on Hive performance, investigating the problem of high memory
> pressure when (a) a table consists of a high number (thousands) of partitions
> and (b) multiple queries run against it concurrently. It turns out that a lot
> of memory is wasted due to data duplication. One source of duplicate strings
> is class org.apache.hadoop.fs.BlockLocation. Its fields such as storageIds,
> topologyPaths, hosts, names, may collectively use up to 6% of memory in my
> benchmark, causing (together with other problematic classes) a huge memory
> spike. Of these 6% of memory taken by BlockLocation strings, more than 5% are
> wasted due to duplication.
> I think we need to add calls to String.intern() in the BlockLocation
> constructor, like:
> {code}
> this.hosts = internStringsInArray(hosts);
> ...
> private void internStringsInArray(String[] sar) {
> for (int i = 0; i < sar.length; i++) {
> sar[i] = sar[i].intern();
> }
> }
> {code}
> String.intern() performs very well starting from JDK 7. I've found some
> articles explaining the progress that was made by the HotSpot JVM developers
> in this area, verified that with benchmarks myself, and finally added quite a
> bit of interning to one of the Cloudera products without any issues.
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