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From "Yonik Seeley (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-672) new merge policy
Date Thu, 14 Sep 2006 17:56:23 GMT
new merge policy
----------------

                 Key: LUCENE-672
                 URL: http://issues.apache.org/jira/browse/LUCENE-672
             Project: Lucene - Java
          Issue Type: Bug
          Components: Index
            Reporter: Ning Li


Today, applications have to open/close an IndexWriter and open/close an
IndexReader directly or indirectly (via IndexModifier) in order to handle a
mix of inserts and deletes. This performs well when inserts and deletes
come in fairly large batches. However, the performance can degrade
dramatically when inserts and deletes are interleaved in small batches.
This is because the ramDirectory is flushed to disk whenever an IndexWriter
is closed, causing a lot of small segments to be created on disk, which
eventually need to be merged.

We would like to propose a small API change to eliminate this problem. We
are aware that this kind change has come up in discusions before. See
http://www.gossamer-threads.com/lists/lucene/java-dev/23049?search_string=indexwriter%20delete;#23049
. The difference this time is that we have implemented the change and
tested its performance, as described below.

API Changes
-----------
We propose adding a "deleteDocuments(Term term)" method to IndexWriter.
Using this method, inserts and deletes can be interleaved using the same
IndexWriter.

Note that, with this change it would be very easy to add another method to
IndexWriter for updating documents, allowing applications to avoid a
separate delete and insert to update a document.

Also note that this change can co-exist with the existing APIs for deleting
documents using an IndexReader. But if our proposal is accepted, we think
those APIs should probably be deprecated.

Coding Changes
--------------
Coding changes are localized to IndexWriter. Internally, the new
deleteDocuments() method works by buffering the terms to be deleted.
Deletes are deferred until the ramDirectory is flushed to disk, either
because it becomes full or because the IndexWriter is closed. Using Java
synchronization, care is taken to ensure that an interleaved sequence of
inserts and deletes for the same document are properly serialized.

We have attached a modified version of IndexWriter in Release 1.9.1 with
these changes. Only a few hundred lines of coding changes are needed. All
changes are commented by "CHANGE". We have also attached a modified version
of an example from Chapter 2.2 of Lucene in Action.

Performance Results
-------------------
To test the performance our proposed changes, we ran some experiments using
the TREC WT 10G dataset. The experiments were run on a dual 2.4 Ghz Intel
Xeon server running Linux. The disk storage was configured as RAID0 array
with 5 drives. Before indexes were built, the input documents were parsed
to remove the HTML from them (i.e., only the text was indexed). This was
done to minimize the impact of parsing on performance. A simple
WhitespaceAnalyzer was used during index build.

We experimented with three workloads:
  - Insert only. 1.6M documents were inserted and the final
    index size was 2.3GB.
  - Insert/delete (big batches). The same documents were
    inserted, but 25% were deleted. 1000 documents were
    deleted for every 4000 inserted.
  - Insert/delete (small batches). In this case, 5 documents
    were deleted for every 20 inserted.

                                current       current          new
Workload                      IndexWriter  IndexModifier   IndexWriter
-----------------------------------------------------------------------
Insert only                     116 min       119 min        116 min
Insert/delete (big batches)       --          135 min        125 min
Insert/delete (small batches)     --          338 min        134 min

As the experiments show, with the proposed changes, the performance
improved by 60% when inserts and deletes were interleaved in small batches.


Regards,
Ning


Ning Li
Search Technologies
IBM Almaden Research Center
650 Harry Road
San Jose, CA 95120

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