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https://issues.apache.org/jira/browse/LUCENE-4062?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Adrien Grand updated LUCENE-4062:
---------------------------------
Attachment: LUCENE-4062.patch
I added the changes we discussed in the patch, replacing fasterButMoreRam=true
by PackedInts.FAST and fasterButMoreRam=false by PackedInts.DEFAULT = 20%. All
Lucene tests passed.
> More fine-grained control over the packed integer implementation that is
> chosen
> -------------------------------------------------------------------------------
>
> Key: LUCENE-4062
> URL: https://issues.apache.org/jira/browse/LUCENE-4062
> Project: Lucene - Java
> Issue Type: Improvement
> Components: core/other
> Reporter: Adrien Grand
> Assignee: Michael McCandless
> Priority: Minor
> Labels: performance
> Fix For: 4.1
>
> Attachments: LUCENE-4062.patch, LUCENE-4062.patch
>
>
> In order to save space, Lucene has two main PackedInts.Mutable implentations,
> one that is very fast and is based on a byte/short/integer/long array
> (Direct*) and another one which packs bits in a memory-efficient manner
> (Packed*).
> The packed implementation tends to be much slower than the direct one, which
> discourages some Lucene components to use it. On the other hand, if you store
> 21 bits integers in a Direct32, this is a space loss of (32-21)/32=35%.
> If you accept to trade some space for speed, you could store 3 of these 21
> bits integers in a long, resulting in an overhead of 1/3 bit per value. One
> advantage of this approach is that you never need to read more than one block
> to read or write a value, so this can be significantly faster than Packed32
> and Packed64 which always need to read/write two blocks in order to avoid
> costly branches.
> I ran some tests, and for 10000000 21 bits values, this implementation takes
> less than 2% more space and has 44% faster writes and 30% faster reads. The
> 12 bits version (5 values per block) has the same performance improvement and
> a 6% memory overhead compared to the packed implementation.
> In order to select the best implementation for a given integer size, I wrote
> the {{PackedInts.getMutable(valueCount, bitsPerValue,
> acceptableOverheadPerValue)}} method. This method select the fastest
> implementation that has less than {{acceptableOverheadPerValue}} wasted bits
> per value. For example, if you accept an overhead of 20%
> ({{acceptableOverheadPerValue = 0.2f * bitsPerValue}}), which is pretty
> reasonable, here is what implementations would be selected:
> * 1: Packed64SingleBlock1
> * 2: Packed64SingleBlock2
> * 3: Packed64SingleBlock3
> * 4: Packed64SingleBlock4
> * 5: Packed64SingleBlock5
> * 6: Packed64SingleBlock6
> * 7: Direct8
> * 8: Direct8
> * 9: Packed64SingleBlock9
> * 10: Packed64SingleBlock10
> * 11: Packed64SingleBlock12
> * 12: Packed64SingleBlock12
> * 13: Packed64
> * 14: Direct16
> * 15: Direct16
> * 16: Direct16
> * 17: Packed64
> * 18: Packed64SingleBlock21
> * 19: Packed64SingleBlock21
> * 20: Packed64SingleBlock21
> * 21: Packed64SingleBlock21
> * 22: Packed64
> * 23: Packed64
> * 24: Packed64
> * 25: Packed64
> * 26: Packed64
> * 27: Direct32
> * 28: Direct32
> * 29: Direct32
> * 30: Direct32
> * 31: Direct32
> * 32: Direct32
> * 33: Packed64
> * 34: Packed64
> * 35: Packed64
> * 36: Packed64
> * 37: Packed64
> * 38: Packed64
> * 39: Packed64
> * 40: Packed64
> * 41: Packed64
> * 42: Packed64
> * 43: Packed64
> * 44: Packed64
> * 45: Packed64
> * 46: Packed64
> * 47: Packed64
> * 48: Packed64
> * 49: Packed64
> * 50: Packed64
> * 51: Packed64
> * 52: Packed64
> * 53: Packed64
> * 54: Direct64
> * 55: Direct64
> * 56: Direct64
> * 57: Direct64
> * 58: Direct64
> * 59: Direct64
> * 60: Direct64
> * 61: Direct64
> * 62: Direct64
> Under 32 bits per value, only 13, 17 and 22-26 bits per value would still
> choose the slower Packed64 implementation. Allowing a 50% overhead would
> prevent the packed implementation to be selected for bits per value under 32.
> Allowing an overhead of 32 bits per value would make sure that a Direct*
> implementation is always selected.
> Next steps would be to:
> * make lucene components use this {{getMutable}} method and let users decide
> what trade-off better suits them,
> * write a Packed32SingleBlock implementation if necessary (I didn't do it
> because I have no 32-bits computer to test the performance improvements).
> I think this would allow more fine-grained control over the speed/space
> trade-off, what do you think?
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