Hi Stuart,
On Mon, Oct 27, 2008 at 7:06 PM, Stuart Halloway
<[EMAIL PROTECTED]> wrote:
>
> Hi all,
>
> The code below implements a Monte Carlo simulation to estimate the
> value of pi. It works, and it was easy to reuse the original single-
> threaded approach across multiple agents.
>
> How idiomatic is this use of agents? Other than bringing in ForkJoin,
> are there other idiomatic ways to parallelize divide-and-conquer in
> Clojure?
>
> Cheers,
> Stuart
>
> (defn in-circle? [[x y]]
> (<= (Math/sqrt (+ (* x x) (* y y))) 1))
>
> (defn random-point []
> [(dec (rand 2)) (dec (rand 2))])
>
> ; take samples, tracking number that land
> ; :in circle and :total number
> (defn sample-for-pi [count]
> (reduce (fn [{in :in total :total} point]
> {:in (if (in-circle? point) (inc in) in)
> :total (inc total)})
> {:in 0 :total 0}
> (take count (repeatedly random-point))))
>
>
> (defn guess-from-samples [samples]
> (assoc samples :guess (/ (* 4.0 (:in samples)) (:total samples))))
>
> ; guess pi by running Monte Carlo simulation count times
> (defn guess-pi [count]
> (guess-from-samples (sample-for-pi count)))
>
> ; guess pi by running Monte Carlo simulation count times
> ; spread across n agents
> (defn guess-pi-agent [n count]
> (let [count (quot count n)
> agents (for [_ (range n)] (agent count))]
> (doseq a agents (send a sample-for-pi))
> (apply await agents)
> (guess-from-samples (reduce (fn [a1 a2]
> {:in (+ (:in @a1) (:in @a2))
> :total (+ (:total @a1) (:total @a2))})
> agents))))
I've not gotten around to playing with the concurrency features of
Clojure yet, so I'm just commenting "theoretically" on your code. In
"guess-pi-agent", you use "await" to wait for the results from the
agents. If one of the agents "dies", won't your code wait forever? I
would think that it would be preferable to use "await-for" (with a
timeout) so that the result can still be guessed based on the samples
that have come back from the other agents. If you plan to use a
variation of this code in a multi-processor example, some degree of
agent failure is even more likely.
--
Bill Clementson
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