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From "Otis Gospodnetic (JIRA)" <>
Subject [jira] Commented: (LUCENE-629) Performance improvement for merging stored, compressed fields
Date Sun, 13 Aug 2006 06:04:15 GMT
    [ ] 
Otis Gospodnetic commented on LUCENE-629:

This looks fine to me, patch applied after a bit of persuading, and unit tests all pass. 
I'll commit this in a bit.
One question, why "...ForMerge" names?  Doesn't this patch really address only compressed
fields?  Wouldn't it make sense to name things (classes, fields, vars) to indicate that? 
Something like "...DontTouchMeImAlreadyCompressed".  Just kidding, but you get the idea.

> Performance improvement for merging stored, compressed fields
> -------------------------------------------------------------
>                 Key: LUCENE-629
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Priority: Minor
>         Attachments: optimized_merge_compressed_fields.patch
> 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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