RFR: JDK-8277175 : Add a parallel multiply method to BigInteger [v7]

[kabutz]

On Thu, 16 Dec 2021 21:37:26 GMT, kabutz <duke at openjdk.java.net> wrote:

> > embarrassingly parallelizable
> 
> Having looked at [embarrassingly parallel](https://en.wikipedia.org/wiki/Embarrassingly_parallel), I'm not certain that this particular problem would qualify. The algorithm is easy to parallelize, but in the end we still have some rather large numbers, so memory will be our primary dominator. I'd expect to see a linear speedup if it was "perfectly parallel", but this does not come close to that.

I ran fibonacci(100_000_000) with multiply() and parallelMultiply(). For multiply() we had:


real	0m25.627s
user	0m26.767s
sys	0m1.197s


and for parallelMultiply() we had


real	0m10.030s
user	1m2.205s
sys	0m2.489s


Thus we are 2.5 times faster on a 1-6-2 machine, but use more than 2x the user time. If it were perfectly parallel, shouldn't we expect to see the parallelMultiply() be close to user time of 26s and real time 4.3s?

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PR: https://git.openjdk.java.net/jdk/pull/6409