RFR 8005311: Add Scalable Updatable Variables, DoubleAccumulator, DoubleAdder, LongAccumulator, LongAdder

[Joe Darcy]

Hello,

I had a question about how the double accumulation logic was intended to 
be used.  I've taken a quick look at the code and it uses 
straightforward "sum = sum + nextValue" code to compute the double sum.  
Summing doubles values with code numerical accuracy is surprisingly 
tricky and if the DoubleAccumulator code is meant for wide use, I'd 
recommend using instead some form of compensated summation:

     http://en.wikipedia.org/wiki/Kahan_summation_algorithm

Thanks,

-Joe

On 1/5/2013 10:10 AM, Chris Hegarty wrote:
> As part of JEP 155 we are proposing to add the following public 
> classes to support Scalable Updatable Variables, DoubleAccumulator, 
> DoubleAdder, LongAccumulator and LongAdder.
>
> These have been written by Doug Lea, with assistance from members of 
> the former JCP JSR-166 Expert Group.
>
> Webrev and javadoc are at:
>   http://cr.openjdk.java.net/~chegar/8005311/ver.00/
>
> Since Doug is the author, I am taking a reviewer/sponsor role.
>
> Here are my initial comments.
>  - There are various places in DoubleAccmulator where there are broken
>    links to #sum ( I think it is just a cut'n'paste error ). These
>    should be #get.
>  - Accumulators
>    {@link #get} may read somewhat better as {@link #get current value} ??
>  - Accumulators
>    Does the 'identity' value need further explanation?
>
> Note: There is one minor change to the implementation. Currently in 
> the jdk8 repo j.u.f.DoubleBinaryOperator defines operateAsDouble. This 
> method has been renamed to applyAsDouble in the lambda/lambda repo. 
> When these changes are sync'ed from lambda/lambda this can be 
> reverted. A similar comment has been added to the code.
>
> -Chris.