Performance improvement for merging stored, compressed fields
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Key: LUCENE-629
URL: http://issues.apache.org/jira/browse/LUCENE-629
Project: Lucene - Java
Issue Type: Improvement
Components: Index
Reporter: Michael Busch
Priority: Minor
Hello everyone,
currently the merging of stored, compressed fields is not optimal for the
following reason: every time a stored, compressed field is being merged, the
FieldsReader uncompresses the data, hence the FieldsWriter has to compress it
again when it writes the merged fields data (.fdt) file. The
uncompress/compress step is unneccessary and slows down the merge performance
significantly.
This patch improves the merge performance by avoiding the uncompress/compress
step. In the following I give an overview of the changes I made:
* Added a new FieldSelectorResult constant named "LOAD_FOR_MERGE" to
org.apache.lucene.document.FieldSelectorResult
* SegmentMerger now uses an FieldSelector to get stored fields from the
FieldsReader. This FieldSelector's accept() method returns the
FieldSelectorResult "LOAD_FOR_MERGE" for every field.
* Added a new inner class to FieldsReader named "FieldForMerge", which
extends org.apache.lucene.document.AbstractField. This class holds the field
properties and its data. If a field has the FieldSelectorResult
"LOAD_FOR_MERGE", then the FieldsReader creates an instance of "FieldForMerge"
and does not uncompress the field's data.
* FieldsWriter checks if the field it is about to write is an instanceof
FieldsReader.FieldForMerge. If true, then it does not compress the field data.
To test the performance I index about 350,000 text files and store the raw text
in a stored, compressed field in the lucene index. I use a merge factor of 10.
The final index has a size of 366MB. After building the index, I optimize it to
measure the pure merge performance.
Here are the performance results:
old version:
* Time for Indexing: 36.7 minutes
* Time for Optimizing: 4.6 minutes
patched version:
* Time for Indexing: 20.8 minutes
* Time for Optimizing: 0.5 minutes
The results show that the index build time improved by about 43%, and the
optimizing step is more than 8x faster.
A diff of the final indexes (old and patched version) shows, that they are
identical. Furthermore, all junit testcases succeeded with the patched version.
Regards,
Michael Busch
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