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From "Tom White (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-7183) WritableComparator.get should not cache comparator objects
Date Tue, 15 Mar 2011 21:02:29 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-7183?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13007180#comment-13007180
] 

Tom White commented on HADOOP-7183:
-----------------------------------

Before HADOOP-6881 the only way to add a WritableComparator to the cache was by explicitly
calling WritableComparator.define(). HADOOP-6881 changed WritableComparator.get() to add a
generic WritableComparator to the comparators cache when there was none registered for the
class. This patch simply restores the previous behaviour.

Making WritableComparator thread-safe by using a thread-local buffer would be a good enhancement
but isn't necessary for this fix.

> WritableComparator.get should not cache comparator objects
> ----------------------------------------------------------
>
>                 Key: HADOOP-7183
>                 URL: https://issues.apache.org/jira/browse/HADOOP-7183
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: Tom White
>            Priority: Blocker
>             Fix For: 0.20.3, 0.21.1, 0.22.0
>
>         Attachments: HADOOP-7183.patch
>
>
> HADOOP-6881 modified WritableComparator.get such that the constructed WritableComparator
gets saved back into the static map. This is fine for stateless comparators, but some comparators
have per-instance state, and thus this becomes thread-unsafe and causes errors in the shuffle
where multiple threads are doing comparisons. An example of a Comparator with per-instance
state is WritableComparator itself.

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