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From "Chuck Williams (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-709) [PATCH] Enable application-level management of IndexWriter.ramDirectory size
Date Thu, 16 Nov 2006 02:18:38 GMT
    [ http://issues.apache.org/jira/browse/LUCENE-709?page=comments#action_12450260 ] 
            
Chuck Williams commented on LUCENE-709:
---------------------------------------

Not synchronizing on the Hashtable, even if using an Enumerator, creates problems as the contents
of the hash table may change during the sizeInBytes() iteration.  Files might be deleted and/or
added to the directory concurrently, causing the size to be computed from an invalid intermediate
state.  Using an Enumerator would cause the invalid value to be returned without an exception,
while using an Iterator instead generates a ConcurrentModificationException.  Synchronizing
on files avoids the problem altogether without much cost as the loop is fast.

Hashtable uses a single class, Hashtable.Enumerator, for both its iterator and its enumerator.
 There are a couple minor differences in the respective methods, such as the above, but not
much.

The issue with RAMFile.length being a long is an issue, but, this bug already exists in Lucene
without sizeInBytes().  See RAMDirectory.fileLength(), which has the same problem now.

I'll submit another verison of the patch that encapsulates RAMFile.length into a sychronized
getter and setter.  It's only used in a few places (RAMDIrectory, RAMInputStream and RAMOutputStream).


> [PATCH] Enable application-level management of IndexWriter.ramDirectory size
> ----------------------------------------------------------------------------
>
>                 Key: LUCENE-709
>                 URL: http://issues.apache.org/jira/browse/LUCENE-709
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 2.0.1
>         Environment: All
>            Reporter: Chuck Williams
>         Attachments: ramDirSizeManagement.patch, ramDirSizeManagement.patch
>
>
> IndexWriter currently only supports bounding of in the in-memory index cache using maxBufferedDocs,
which limits it to a fixed number of documents.  When document sizes vary substantially, especially
when documents cannot be truncated, this leads either to inefficiencies from a too-small value
or OutOfMemoryErrors from a too large value.
> This simple patch exposes IndexWriter.flushRamSegments(), and provides access to size
information about IndexWriter.ramDirectory so that an application can manage this based on
total number of bytes consumed by the in-memory cache, thereby allow a larger number of smaller
documents or a smaller number of larger documents.  This can lead to much better performance
while elimianting the possibility of OutOfMemoryErrors.
> The actual job of managing to a size constraint, or any other constraint, is left up
the applicatation.
> The addition of synchronized to flushRamSegments() is only for safety of an external
call.  It has no significant effect on internal calls since they all come from a sychronized
caller.

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