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From "Mithun Radhakrishnan (Jira)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-19261) Avro SerDe's InstanceCache should not be synchronized on retrieve
Date Fri, 13 Sep 2019 23:39:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-19261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16929610#comment-16929610
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Mithun Radhakrishnan commented on HIVE-19261:
---------------------------------------------

Hello, [~shengzhixia], et al.

This looks like a good fix, generally. A couple of suggestions:
 # Please consider the rephrasing the ending of {{InstanceCache::retrieve()}} as follows:
 ## {code:java}
Instance newInstance = makeInstance(hv, seenSchemas);
instance = cache.putIfAbsent(hv, newInstance);
return instance == null? newInstance : instance;{code}
 # Please remove the documentation regarding {{ConcurrentHashMap::computeIfAbsent()}}; it
won't work here. {{makeInstance()}} lands up making reentrant calls to {{InstanceCache::retrieve()}}.
{{computeIfAbsent()}} requires that the {{cache}} not be modified within a single call to
the {{putIfAbsent()}} lambda (i.e. {{hv -> makeInstance(hv, seenSchemas)}}. As such, this
will cause {{retrieve()}} to hang on the first reentrant call.

> Avro SerDe's InstanceCache should not be synchronized on retrieve
> -----------------------------------------------------------------
>
>                 Key: HIVE-19261
>                 URL: https://issues.apache.org/jira/browse/HIVE-19261
>             Project: Hive
>          Issue Type: Improvement
>            Reporter: Fangshi Li
>            Assignee: Fangshi Li
>            Priority: Major
>         Attachments: HIVE-19261.1.patch
>
>
> In HIVE-16175, upstream made a patch to fix the thread safety issue in AvroSerDe's InstanceCache.
This fix made the retrieve method in InstanceCache synchronized. While it should make InstanceCache
thread-safe, making retrieve synchronized for the cache can be expensive in highly concurrent environment
like Spark, as multiple threads need to be synchronized on entering the entire retrieve method.
> We are proposing another way to fix this thread safety issue by making the underlying
map of InstanceCache as ConcurrentHashMap. Ideally, we can use atomic computeIfAbsent in
the retrieve method to avoid synchronizing the entire method.
> While computeIfAbsent is only available on java 8 and java 7 is still supported in Hive,
> we use a pattern to simulate the behavior of computeIfAbsent. In the future, we should
move to computeIfAbsent when Hive requires java 8.



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