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From "Adrien Grand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-4062) More fine-grained control over the packed integer implementation that is chosen
Date Fri, 29 Jun 2012 15:33:44 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-4062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13403971#comment-13403971
] 

Adrien Grand commented on LUCENE-4062:
--------------------------------------

This is very interesting!

Results computed on mars are very strange so I think we should not take them into account
to make decisions...

bq. When should we stop optimizing?

This is a relevant question. I think there is no problem going further provided that we have
evidence that the changes improve performance in most cases and that the code remains maintainable.
For example, it is probably not necessary to work on specialized Packed64 for the 2^n bits
cases since there are direct impls for the 8, 16, 32 and 64 bits cases and since the new Packed64SingleBlock
impl seems to be as fast for 1, 2 and 4 bits per value.

However, I think it makes sense to replace Packed64SingleBlock with the inheritance-based
impl since it seems faster than the current impl and than Packed64. I will open an issue if
the graphs computed on your core i7 computer confirm this trend.
                
> 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: Adrien Grand
>            Priority: Minor
>              Labels: performance
>             Fix For: 4.0, 5.0
>
>         Attachments: LUCENE-4062-2.patch, LUCENE-4062.patch, LUCENE-4062.patch, LUCENE-4062.patch,
LUCENE-4062.patch, LUCENE-4062.patch, LUCENE-4062.patch, LUCENE-4062.patch, Packed64SingleBlock.java,
Packed64Strategy.java, Packed64calc.java, PackedIntsBenchmark.java, PackedIntsBenchmark.java,
PackedIntsBenchmark.java, measurements_te_graphs.pdf, measurements_te_i7.txt, measurements_te_p4.txt,
measurements_te_xeon.txt
>
>
> 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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