[Haskell-cafe] why is Random in System?

Ryan Newton rrnewton at gmail.com
Fri Aug 19 20:03:26 CEST 2011

> Yep, but don't conflate determinism with splitting. In the imperative
> world, you normally know how many CPUs you have, so you initialize one PRNG
> per CPU, and simply go from there; there's no need for splitting. In the
> parallel community, people are going out of their way to *avoid*
>  splitting.

I'm having trouble thinking of scenarios where per-CPU does the trick.  Do
you mean one per pthread rather than one per CPU?

In the Cilk case, you've got to deal with work stealing of course.  So you
want rand() to generate a result that is determined by the current
stack-frame's position in the binary-tree of spawns.  In the work I was
referring to:


... they try a bunch of different methods and I can't remember if any of
them split the RNG "eagerly" as they go down the spawn tree or if they just
record the tree-index on the way down and then read it out when they
generated randoms.  (I think the latter.)

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