[cp-patches] RFC: changes to java.lang.Integer, Long...

Ian Rogers rogers.email at gmail.com
Mon Apr 14 20:25:58 UTC 2008


David Daney wrote:
> Ian Rogers wrote:
>> Hi,
>>
>> please give your comments on the attached patch. It tries to reduce 
>> the size of char[] for strings used to hold numbers. It changes 
>> Float/Double equals to use bit based comparisons rather than 
>> division. It increases the use of valueOf methods. It adds a cache of 
>> values from -128 to 127 for Long. It adds a cache of the values of 
>> zero and one to Float and Double.
>>
>> The string size is an estimate. For decimal numbers it will divide 
>> the value repeatedly by 8, causing the string length to be over 
>> estimated by a character for values like 999. This string size is 
>> still better than the current estimate of 33 characters. It also 
>> avoids the use of division (shifts are used) and/or lookup tables.
>>
>
> I would like to know your motivation for doing this.  Do you have any 
> evidence that this will reduce memory usage and speed up real 
> applications?
>
>
> That said, in our gcj-3.4 based application, we had to create a cache 
> of Integers because we were creating large numbers of them all with a 
> small set of values.
>
> So in principle this could be a good approach, but I don't know if we 
> can assume that there is universal benefit from a patch like this.  
> Can you point to any benchmarks where this helps?
>
>
> Thanks,
> David Daney
Hi David,

I'm having a crack down on wasted memory in the Jikes RVM.

For DaCapo fop (single iteration) there are 270 and 977 occurrences of  
Double 0 and 1 and 20 occurrences of other Doubles. On the other hand 
DaCapo bloat has very few 0 and 1 values. My motivation to cache these, 
other than fop, is that they exist as bytecodes (fconst0/fconst1 and 
dconst0/dconst1, although I'm ignoring fconst2 and dconst2). We already 
cache Integers in the intCache. I do extend this concept to Long, as is 
done in OpenJDK, and to Float and Double.

Currently we always allocated 33 char arrays to hold the string value, 
this is 4.625 the size of the minimum object in the RVM. In the case of 
a single character string, 18.86% of Integer strings in DaCapo bloat, 
this code doesn't allocate any char arrays. For other integers the char 
array is reduced to either the exact or (20% of the time for decimal 
values)  1 character longer char arrays. This is at the cost of up to 32 
compares, branches and shifts. For DaCapo bloat a little under 50% of 
integer strings created are for values between -128 and 127.

So the trade offs in the code are, slower Float/Double valueOf code, but 
fewer Float and Double objects (hopefully improving GC). A small time to 
calculate string sizes vs smaller strings and less GC pressure.

For the Jikes RVM we measure performance 4 times a day [1], I introduced 
this patch in r14113 and there are no peaks or troughs that appear at 
this point. Given the patch is performance neutral but saves memory 
(although not improving GC performance for the RVM markedly) I think 
it's worth including. GC is less than 6% of execution time, so time 
saved may be difficult to measure in the bigger picture (unless it 
pushes you under or over a particular threshold).

Regards,
Ian

[1] 
http://jikesrvm.anu.edu.au/cattrack/results/rvmx86lnx32.anu.edu.au/perf/3437/performance_report 




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