ForkJoin wakeup update

[Aleksey Shipilev]

On 10/18/2012 10:03 PM, Doug Lea wrote:
> Thanks! This leads to a much more precise version of
> my main observations about FJ in other contexts:
> 
> * The break-even point (in terms of elements * cost per element)
> can be reduced by about a factor of three at the expense of wasting
> the world's power by keeping parallel worker threads spinning.
> Which we really do not want to do.

Yeah, it also gives the upper limit on what we can do in parallel.
Comparing this against sequential version, sequential case is doing the
same thing in 450us (since we are at break-even point), which actually
means we throw 24x more threads in, but get only 3x improvement, so then
the rough estimate for sequential work we do is 30%. Gotta churn out
that as well.

> * Short of that, the case for being only a bit more aggressive
> (and possibly wasteful) in initial startup wakeups isn't all that
> compelling but might be worth more tuning effort.

It might very well turn out the quicker rampup does compelling
improvements on larger machines, 24 hardware threads is nothing these
days. Will look around for 4x10x2 and greater machines.

> * The main systems-level goal should be to find ways to make
> wakeups faster. Possibly entailing support of some sort of bulk
> signalling facilities?

Yes, the ball is in Dave's court here.

-Aleksey.