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

[Doug Lea]

On 01/07/13 14:07, Joe Darcy wrote:
> 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
>

I'm sympathetic...
Complete lack of control over arithmetic issues (here and for
plain atomics) led me to resist offering Double versions for
years. But many people are content with the scalability vs
numerical stability tradeoffs intrinsic here (and I think
unsolvable). I suppose it could stand an explicit disclaimer
rather than the implicit one there now.

How about: "The order of accumulation of sums across threads is
uncontrolled. Numerical stability of results is not guaranteed
when values of substantially different orders of magnitude are
combined"

Or something better?

-Doug