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Toke Eskildsen commented on LUCENE4062:

+1 for replacing the Packed64SingleBlock with the optimized version. It is consistently better.
I have updated the charts at http://ekot.dk/misc/packedints/ with i7 measurements. For the
three noni7based Intel processors, Packed64SingleBlock seems clearcut for the "Best possible
performance without going allout with Direct".
I tried running two tests in parallel on teprime (see http://ekot.dk/misc/packedints/teprime_parallel.html)
and got very consistent results:
* For set, contiguous strategy is faster than or equal to padded for bpvs 312 and 1621
(with bpv 1 and 2 being hard to measure). Since padded only supports bpv 110, 12, 16, 21
& 32 with nonconforming bpvs rounded up, this effectively means that there is zero gain
in using padded over contiguous with regard to set on that machine.
* For get, the picture is the same, except for process 2, where 1721 was slower for contiguous
than for padded. Sigh. I should of course have started 3 processes so voting would have been
easier. The same pattern, although noisier, can be seen on mars.
My preliminary conclusion for i7processors is thus that Packed64Strategy is the right choice
for "Best possible performance without going allout with Direct". I am getting very curious
about the untested AMD architecture now.
A byproduct of the teprime parallel test is that the amount of cache seems to matter little
when it comes to selecting the most fitting implementation. Thank $diety for small favors.
> 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, 5.0
>
> Attachments: LUCENE40622.patch, LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.patch,
LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.patch, LUCENE4062.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 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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