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From "Robert Muir (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2903) Improvement of PForDelta Codec
Date Wed, 02 Feb 2011 01:00:38 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2903?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12989480#comment-12989480
] 

Robert Muir commented on LUCENE-2903:
-------------------------------------

Just curious: why does the patch remove BulkVint's optimization for blocks of all 1's (it
writes a 0-byte header only in this case) ?

{noformat}
+          allOnes = false;
           if (allOnes) {
             // the most common int pattern (all 1's)
             // write a special header (numBytes=0) for this case.
{noformat}

This is an important optimization I think: besides the fact its the most common bitpattern[1],
its efficient: a single compare-to-zero for the entire block of 128 ints, and it takes care
of several worst-cases for vint: blocks of all 1 docdeltas (something more commonly seen in
structured data, but still the most common pattern in unstructured, stopwordish things), and
all 1 freqs (e.g. you should have omitTF'ed). Depending on block size this significantly reduces
the .doc/.freq files for vint, and still helps in the pure unstructured case (I measured this
with luceneutil).

[1] http://portal.acm.org/citation.cfm?id=1712668

Furthermore, I was thinking that along the lines of this allOnes trick, we could evaluate
an alternative to the "Sep" file layout: instead at least we should consider interleaving
.doc and .freq (block of doc deltas, block of freqs).
With this interleaved layout, something only interested in doc deltas can just read the freq
byte header and skip these bytes to bypass all the freqs... omitTF is then implemented automatically
for a lot of cases (though this wouldn't be equivalent to lucene's manually-set omitTFAP today,
as positions would still exist). If you did manually set omitTF, we could arguably just write
this same 0 byte header for freq blocks, which means all 1 freqs, and not have so much specialization
and different codepaths.


> Improvement of PForDelta Codec
> ------------------------------
>
>                 Key: LUCENE-2903
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2903
>             Project: Lucene - Java
>          Issue Type: Improvement
>            Reporter: hao yan
>         Attachments: LUCENE_2903.patch
>
>
> There are 3 versions of PForDelta implementations in the Bulk Branch: FrameOfRef, PatchedFrameOfRef,
and PatchedFrameOfRef2.
> The FrameOfRef is a very basic one which is essentially a binary encoding (may result
in huge index size).
> The PatchedFrameOfRef is the implmentation based on the original version of PForDelta
in the literatures.
> The PatchedFrameOfRef2 is my previous implementation which are improved this time. (The
Codec name is changed to NewPForDelta.).
> In particular, the changes are:
> 1. I fixed the bug of my previous version (in Lucene-1410.patch), where the old PForDelta
does not support very large exceptions (since
> the Simple16 does not support very large numbers). Now this has been fixed in the new
LCPForDelta.
> 2. I changed the PForDeltaFixedIntBlockCodec. Now it is faster than the other two PForDelta
implementation in the bulk branch (FrameOfRef and PatchedFrameOfRef). The codec's name is
"NewPForDelta", as you can see in the CodecProvider and PForDeltaFixedIntBlockCodec.
> 3. The performance test results are:
> 1) My "NewPForDelta" codec is faster then FrameOfRef and PatchedFrameOfRef for almost
all kinds of queries, slightly worse then BulkVInt.
> 2) My "NewPForDelta" codec can result in the smallest index size among all 4 methods,
including FrameOfRef, PatchedFrameOfRef, and BulkVInt, and itself)
> 3) All performance test results are achieved by running with "-server" instead of "-client"

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