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https://issues.apache.org/jira/browse/LUCENE-2905?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Muir updated LUCENE-2905:
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Attachment: LUCENE-2905_skipIntervalMin.patch
here's a patch with what Mike suggested, moving the skipping stuff private to
the codecs, and separating the interval from the minimum df necessary to index
skip data.
i also added the 'dont skip when close' opto to Sep and Preflex codecs (since i
neglected to do this and only did Standard).
I kept all parameters the same (I think we should benchmark etc before
changing) but I did some experiments with a fairly large skipMinimum and it
looked promising. I think we later (after we have good benchmarks) set this
reasonable, and maybe bump skipInterval for Sep codec too, especially if we
improve its 'pendingPositions' consuming to balance it out.
So, I think we should apply this little patch as-is as a small step.
> Sep codec writes insane amounts of skip data
> --------------------------------------------
>
> Key: LUCENE-2905
> URL: https://issues.apache.org/jira/browse/LUCENE-2905
> Project: Lucene - Java
> Issue Type: Bug
> Reporter: Robert Muir
> Fix For: Bulk Postings branch
>
> Attachments: LUCENE-2905_skipIntervalMin.patch
>
>
> Currently, even if we use better compression algorithms via Fixed or Variable
> Intblock
> encodings, we have problems with both performance and index size versus
> StandardCodec.
> Consider the following numbers:
> {noformat}
> standard:
> frq: 1,862,174,204 bytes
> prx: 1,146,898,936 bytes
> tib: 541,128,354 bytes
> complete index: 4,321,032,720 bytes
> bulkvint:
> doc: 1,297,215,588 bytes
> frq: 725,060,776 bytes
> pos: 1,163,335,609 bytes
> tib: 729,019,637 bytes
> complete index: 5,180,088,695 bytes
> simple64:
> doc: 1,260,869,240 bytes
> frq: 234,491,576 bytes
> pos: 1,055,024,224 bytes
> skp: 473,293,042 bytes
> tib: 725,928,817 bytes
> complete index: 4,520,488,986 bytes
> {noformat}
> I think there are several reasons for this:
> * Splitting into separate files (e.g. postings into .doc + .freq).
> * Having to store both a relative delta to the block start, and an offset
> into the block.
> * In a lot of cases various numbers involved are larger than they should be:
> e.g. they are file pointer deltas, but blocksize is fixed...
> Here are some ideas (some are probably stupid) of things we could do to try
> to fix this:
> Is Sep really necessary? Instead should we make an alternative to Sep,
> Interleaved? that interleaves doc and freq blocks (doc,freq,doc,freq) into
> one file? the concrete impl could implement skipBlock() for when they only
> want docdeltas: e.g. for Simple64 blocks on disk are fixed size so it could
> just skip N bytes. Fixed Int Block codecs like PFOR and BulkVint just read
> their single numBytes header they already have today, and skip numBytes.
> Isn't our skipInterval too low? Most of our codecs are using block sizes such
> as 64 or 128, so a skipInterval of 16 seems a little overkill.
> Shouldn't skipInterval not even be a final constant in SegmentWriteState, but
> instead completely private to the codec?
> For block codecs, doesn't it make sense for them to only support skipping to
> the start of a block? Then, their skip pointers dont need to be a combination
> of delta + upto, because upto is always zero. What would we have to modify in
> the bulkpostings api for jump() to work with this?
> For block codecs, shouldn't skipInterval then be some sort of divisor, based
> on block size (maybe by default its 1, meaning we can skip to the start of a
> every block)
> For codecs like Simple64 that encode fixed length frames, shouldnt we use
> 'blockid' instead of file pointer so that we get smaller numbers? e.g.
> simple64 can do blockid * 8 to get to the file pointer.
> Going along with the blockid concept, couldnt pointers in the terms dict be
> blockid deltas from the index term, instead of fp deltas? This would be
> smaller numbers and we could compress this metadata better.
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