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From "Misha Dmitriev (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11383) String duplication in org.apache.hadoop.fs.BlockLocation
Date Fri, 19 May 2017 19:37:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16017902#comment-16017902
] 

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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