Hi Todd,
I'll recommend you to open this up as a separate issue, seeing how
this thread is meant to specifically discuss changes in the 'equiv'
branch. Your examples makes sense, as a 2 item collection cannot open
up more than 2 threads, and a 20 item collection, will open n + 2
threads as promised by pmap.
Lau
Todd wrote:
> (running clojure 1.2 snapshot)
>
> Q1: Why does pmap use the number of available processors + 2? I would
> have thought it would just use the number of avail processors...
>
> Q2: Could someone clear up my misunderstanding of pmap w/ respect to the
> code snippets below? Pmap doesn't seem to be limiting the number of
> threads to the number of processors + 2...
>
> I've created an anon func that does't return, so I wouldn't expect the
> pmap step function to advance beyond 4 (I have 2 processors):
>
> #1: Limit pmap to # of processors
> ----------------------------------
>
> user=> (pmap #(while true (do (println "Thread: " (.getId
> (Thread/currentThread)) "item: " %)(Thread/sleep 500))) (range
> (.availableProcessors (Runtime/getRuntime))))
> Thread: 12 item: 0
> (Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
> Thread: 13 item: 1
> Thread: 12 item: 0
>
> --> just two threads running, as expected
>
>
> #2: Limit pmap to # of processors * 10
> --------------------------------------
>
> user=> (pmap #(while true (do (println "Thread: " (.getId
> (Thread/currentThread)) "item: " %)(Thread/sleep 500))) (range (* 10
> (.availableProcessors (Runtime/getRuntime)))))
> Thread: Thread: 12 item: 0
> (Thread: 25 item: 13
> Thread: 24 item: 12
> Thread: 23 item: 11
> Thread: 22 item: 10
> Thread: 21 item: 9
> Thread: 20 item: 8
> Thread: 19 item: 7
> Thread: 18 item: 6
> Thread: 17 item: 5
> Thread: 16 item: 4
> Thread: 15 item: 3
>
> --> In this short snippet, you can see > 4 threads running...expected?
>
>
> toddg=> (source pmap)
> (defn pmap
> "Like map, except f is applied in parallel. Semi-lazy in that the
> parallel computation stays ahead of the consumption, but doesn't
> realize the entire result unless required. Only useful for
> computationally intensive functions where the time of f dominates
> the coordination overhead."
> {:added "1.0"}
> ([f coll]
> (let [n (+ 2 (.. Runtime getRuntime availableProcessors))
> rets (map #(future (f %)) coll)
> step (fn step [[x & xs :as vs] fs]
> (lazy-seq
> (if-let [s (seq fs)]
> (cons (deref x) (step xs (rest s)))
> (map deref vs))))]
> (step rets (drop n rets))))
> <snip>
>
> -Todd
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