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Adrien Grand commented on LUCENE4062:

The x axis is the number of bits per value while the y axis is the number of values that are
read or written per second. For every bitsPerValue and bitpacking scheme, I took the impl
with the lowest working bitsPerValue. (For example, bitsPerValue=19 would give a Direct32,
a Packed64(bitsPerValue=19), a Packed8ThreeBlocks(24 bits per value) and a Packed64SingleBlock(bitsPerValue=21)).
There are 4 lines because we currently have 4 different bitpacking schemes.
In the two first cases, values are read at random offsets while the two bulk tests read/write
a large number of values sequentially. I didn't want to test {{System.arraycopy}} against
a naive forloop, I just noticed that {{Direct64}} bulk operations didn't use {{arraycopy}},
so I fixed that and added a few words about it so that people understand why the throughput
increases when bitsPerValue > 32, which is counterintuitive.
> More finegrained control over the packed integer implementation that is chosen
> 
>
> Key: LUCENE4062
> URL: https://issues.apache.org/jira/browse/LUCENE4062
> Project: Lucene  Java
> Issue Type: Improvement
> Components: core/other
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
> Labels: performance
> Fix For: 4.0
>
> Attachments: LUCENE40622.patch, LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.patch,
LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.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 memoryefficient 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 (3221)/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 2226 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 tradeoff
better suits them,
> * write a Packed32SingleBlock implementation if necessary (I didn't do it because I
have no 32bits computer to test the performance improvements).
> I think this would allow more finegrained control over the speed/space tradeoff, what
do you think?

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