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Date: Sun, 28 Nov 2010 10:57:37 -0500 (EST)
From: "Yonik Seeley (JIRA)" <jira@apache.org>
To: dev@lucene.apache.org
Subject: [jira] Commented: (LUCENE-2779) Use ConcurrentHashMap in
 RAMDirectory
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    [ https://issues.apache.org/jira/browse/LUCENE-2779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12964565#action_12964565 ] 

Yonik Seeley commented on LUCENE-2779:
--------------------------------------

The generic problem with optimizing something is the risk of introducing bugs (so hopefully we're optimizing where it counts).

At a quick glance, I see:
- a race in listAll() (as earwin indicated) that can cause an AIOB exception
- a race in createOutput that can lead to an incorrect directory size (multiple threads can subtract the size of the same file being overwritten).  This probably isn't an issue with how Lucene uses RAMDirectory.

Changing how concurrency works is rarely easy ;-)

> Use ConcurrentHashMap in RAMDirectory
> -------------------------------------
>
>                 Key: LUCENE-2779
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2779
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Store
>            Reporter: Shai Erera
>            Assignee: Shai Erera
>            Priority: Minor
>             Fix For: 3.1, 4.0
>
>         Attachments: LUCENE-2779.patch
>
>
> RAMDirectory synchronizes on its instance in many places to protect access to map of RAMFiles, in addition to updating the sizeInBytes member. In many places the sync is done for 'read' purposes, while only in few places we need 'write' access. This looks like a perfect use case for ConcurrentHashMap
> Also, syncing around sizeInBytes is unnecessary IMO, since it's an AtomicLong ...
> I'll post a patch shortly.

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