It is pretty common to get a slowdown when moving from map to pmap. It just means that the thread scheduling overhead is greater than the gain from parallelizing the code. The behavior might be very different w/ larger data sets, or w/ more CPU's. It is often best to leave parallelism as an option to the client of your library. Are you using the latest build? pmap now uses Java futures and they run in a cached thread pool. If so, try running the algorithm a few times and see if it speeds up as the thread pool winds up.
On Fri, Mar 6, 2009 at 1:31 PM, Dimiter malkia Stanev <mal...@gmail.com>wrote: > > Hi guys, anyone has an insight into the problem I'm running into? > > On Mar 4, 2:27 am, "Dimiter \"malkia\" Stanev" <mal...@gmail.com> > wrote: > > Hi guys, > > > > In the example below, if map is replaced with pmap, it goes twice > > slower on my MBP (2 CPUs). > > > > I believe it's probably the (reduce + ...) causing it, but I can't > > explain it. > > > > Version with map: > > user> (time (pnpolytest)) > > "Elapsed time: 3903.533 msecs" > > 6000000 > > > > Version with pmap: > > user> (time (pnpolytest)) > > "Elapsed time: 7995.565 msecs" > > > > What I'm doing wrong here? > > > > (defn- pnpoly [npol #^floats xp #^floats yp x y] > > (let [npol (int npol) > > x (float x) > > y (float y)] > > (loop [r (int 0) > > i (int 0) > > xi (aget xp (dec npol)) > > yi (aget yp (dec npol))] > > (if (< i npol) > > (let [xj xi xi (aget xp i) > > yj yi yi (aget yp i)] > > (if (or (= (> yi y) (> yj y)) > > (>= x (+ xi (/ (* (- xj xi) > > (- y yi)) > > (- yj yi))))) > > (recur (bit-not r) (inc i) xi yi) > > (recur r (inc i) xi yi))) > > (- r))))) > > > > (defn pnpolytest [] > > (let [xp > > (float-array > > [+0.0 +1.0 +1.0 +0.0 +0.0 > > +1.0 -0.5 -1.0 -1.0 -2.0 > > -2.5 -2.0 -1.5 -0.5 +1.0 > > +1.0 +0.0 -0.5 -1.0 -0.5]) > > yp > > (float-array > > [+0.0 +0.0 +1.0 +1.0 +2.0 > > +3.0 +2.0 +3.0 0.0 -0.5 > > -1.0 -1.5 -2.0 -2.0 -1.5 > > -1.0 -0.5 -1.0 -1.0 -0.5]) > > npol 20] > > (reduce + > > ;; Replace the nex map with pmap, > > ;; and things slow down > > (map (fn [_] > > (+ (pnpoly npol xp yp 0.5 0.5) > > (pnpoly npol xp yp 0.5 1.5) > > (pnpoly npol xp yp -0.5 1.5) > > (pnpoly npol xp yp 0.75 2.25) > > (pnpoly npol xp yp 0.0 2.01) > > (pnpoly npol xp yp -0.5 2.5) > > (pnpoly npol xp yp -1.0 -0.5) > > (pnpoly npol xp yp -1.5 0.5) > > (pnpoly npol xp yp -2.25 -1.0) > > (pnpoly npol xp yp 0.5 -0.25) > > (pnpoly npol xp yp 0.5 -1.25) > > (pnpoly npol xp yp -0.5 -2.5))) > > (range 0 1000000))))) > > > > Thanks, > > Dimiter "malkia" Stanev > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
