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From "Adrien Grand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4509) Make CompressingStoredFieldsFormat the new default StoredFieldsFormat impl
Date Fri, 26 Oct 2012 20:45:12 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13485187#comment-13485187
] 

Adrien Grand commented on LUCENE-4509:
--------------------------------------

bq. How would this work with laaaarge documents that might be > 16KB in size?

Actually 16kB is the minimum size of an uncompressed chunk of documents. CompressingStoredFieldsWriter
fills a buffer with documents until its size is >= 16kb, compresses it and then flushes
to disk. If all documents are greater than 16kB then all chunks will contain exactly one document.

It also means you could end up having a chunk that is made of 15 documents of 1kb and 1 document
of 256kb. (And in this case there is no performance problem for the 15 first documents given
that uncompression stops as soon as enough data has been uncompressed.)

bq. Does this mean with the default CompressingStoredFieldsIndex setting that now he pays
12-bytes/doc in RAM (because docsize > blocksize)? If so, lets think of ways to optimize
that case.

Probably less than 12. The default CompressingStoredFieldsIndex impl uses two packed ints
arrays of size numChunks (the number of chunks, <= numDocs). The first array stores the
doc ID of the first document of the chunk while the second array stores the start offset of
the chunk of documents in the fields data file.

So if your fields data file is fdtBytes bytes, the actual memory usage is ~ {{numChunks *
(ceil(log2(numDocs)) + ceil(log2(fdtBytes))) / 8}}.

For example, if there are 10M documents of 16kB (fdtBytes ~= 160GB), we'll have numChunks
== numDocs and a memory usage per document of (24 + 38) / 8 = 7.75 => ~ 77.5 MB of memory
overall.

bq. 100GB of compressed stored fields == 6.25M index entries == 75MB RAM

Thanks for the figures, Yonik! Did you use RamUsageEstimator to compute the amount of used
memory?
                
> Make CompressingStoredFieldsFormat the new default StoredFieldsFormat impl
> --------------------------------------------------------------------------
>
>                 Key: LUCENE-4509
>                 URL: https://issues.apache.org/jira/browse/LUCENE-4509
>             Project: Lucene - Core
>          Issue Type: Wish
>          Components: core/store
>            Reporter: Adrien Grand
>            Priority: Minor
>
> What would you think of making CompressingStoredFieldsFormat the new default StoredFieldsFormat?
> Stored fields compression has many benefitsĀ :
>  - it makes the I/O cache work for us,
>  - file-based index replication/backup becomes cheaper.
> Things to know:
>  - even with incompressible data, there is less than 0.5% overhead with LZ4,
>  - LZ4 compression requires ~ 16kB of memory and LZ4 HC compression requires ~ 256kB,
>  - LZ4 uncompression has almost no memory overhead,
>  - on my low-end laptop, the LZ4 impl in Lucene uncompresses at ~ 300mB/s.
> I think we could use the same default parameters as in CompressingCodec :
>  - LZ4 compression,
>  - in-memory stored fields index that is very memory-efficient (less than 12 bytes per
block of compressed docs) and uses binary search to locate documents in the fields data file,
>  - 16 kB blocks (small enough so that there is no major slow down when the whole index
would fit into the I/O cache anyway, and large enough to provide interesting compression ratiosĀ ;
for example Robert got a 0.35 compression ratio with the geonames.org database).
> Any concerns?

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