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From "Prasanth Jayachandran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-16079) HS2: high memory pressure due to duplicate Properties objects
Date Thu, 02 Mar 2017 08:40:45 GMT

    [ https://issues.apache.org/jira/browse/HIVE-16079?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15891812#comment-15891812
] 

Prasanth Jayachandran commented on HIVE-16079:
----------------------------------------------

Heads up. Currently we are not enforcing jdk8 as minimum required version. Using BiConsumer
might fail builds on jdk7. There is some discussion to move hive to jdk8 as minimum required
version but until then this will keep breaking jdk7 builds. Travis will show failures as it
builds on both jdk7 and 8. 

> HS2: high memory pressure due to duplicate Properties objects
> -------------------------------------------------------------
>
>                 Key: HIVE-16079
>                 URL: https://issues.apache.org/jira/browse/HIVE-16079
>             Project: Hive
>          Issue Type: Improvement
>          Components: HiveServer2
>            Reporter: Misha Dmitriev
>            Assignee: Misha Dmitriev
>         Attachments: HIVE-16079.01.patch, hs2-crash-2000p-500m-50q.txt
>
>
> I've created a Hive table with 2000 partitions, each backed by two files, with one row
in each file. When I execute some number of concurrent queries against this table, e.g. as
follows
> {code}
> for i in `seq 1 50`; do beeline -u jdbc:hive2://localhost:10000 -n admin -p admin -e
"select count(i_f_1) from misha_table;" & done
> {code}
> it results in a big memory spike. With 20 queries I caused an OOM in a HS2 server with
-Xmx200m and with 50 queries - in the one with -Xmx500m.
> I am attaching the results of jxray (www.jxray.com) analysis of a heap dump that was
generated in the 50queries/500m heap scenario. It suggests that there are several opportunities
to reduce memory pressure with not very invasive changes to the code. One (duplicate strings)
has been addressed in https://issues.apache.org/jira/browse/HIVE-15882 In this ticket, I am
going to address the fact that almost 20% of memory is used by instances of java.util.Properties.
These objects are highly duplicate, since for each partition each concurrently running query
creates its own copy of Partion, PartitionDesc and Properties. Thus we have nearly 100,000
(50 queries * 2,000 partitions) Properties in memory. By interning/deduplicating these objects
we may be able to save perhaps 15% of memory.
> Note, however, that if there are queries that mutate partitions, the corresponding Properties
would be mutated as well. Thus we cannot simply use a single "canonicalized" Properties object
at all times for all Partition objects representing the same DB partition. Instead, I am going
to introduce a special CopyOnFirstWriteProperties class. Such an object initially internally
references a canonicalized Properties object, and keeps doing so while only read methods are
called. However, once any mutating method is called, the given CopyOnFirstWriteProperties
copies the data into its own table from the canonicalized table, and uses it ever after.



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