Interesting. I'll try some of your suggested tests to see if my pmapall all is behaving better than I thought.
Does your pmap-pool permit nesting? (That is, does it permit passing pmap-pool a function which itself calls pmap-pool?). If so then that would be a reason to prefer it over my pmapall. Thanks, -Lee On Oct 10, 2011, at 9:43 PM, j-g-faustus wrote: > I made an alternative implementation using a thread pool and a queue, based > on the example at > http://clojure.org/concurrent_programming > > In short, your pmapall and the pool-based implementation (below) both give > approximately > perfect scaling on my 4/8-core system (Intel i7 920 and HT). > Both give close to full load on all cores and a factor 4.4 speedup compared > to single threaded. > This seems about right, the CPU has four physical cores and get a few percent > extra performance > from the virtual cores, so the speedup is approximately linear with the > number of cores. > > pmap-pool may be a tiny bit faster than the pmapall, but they are so close > that I can't > really tell. > > It is possible that there is some sort of synchronization overhead on your > 48-core machine. > 95% of the tasks are practically noops, after all - just the cost of a single > function call. > There are only 48 tasks in your test that actually require computation, so > each > core will do a bunch of noops and perhaps one "real" task. > > In real time, a single i7 920 runs the test just as fast as your 48 cores. I > don't expect that's > representative for what your 48 cores can do. > > I suggest > * Increase the test size and/or the density of "heavy" tasks. > * Let the "light" tasks do a bit more computation, at least enough to pay for > the > overhead of calling them. > * Start with a smaller number of threads, and see where it stops scaling > linearly. > > > Threadpool/queue-based implementation: > > (import '(java.util.concurrent Executors)) > (defn pmap-pool [f coll] > (let [queue (ref coll) ;; shared queue of work units > nthreads (.availableProcessors (Runtime/getRuntime)) > pool (Executors/newFixedThreadPool nthreads) > tasks (map (fn [_] > (fn [] ; one task per thread > (let [local-res (atom [])] ;; collect results per > thread to minimize synchronization > (while (seq @queue) > ;; queue may be emptied between 'while' > ;; and 'dosync'. > (when-let [wu (dosync > ;; grab work unit, update > queue > (when-let [w (first @queue)] > (alter queue next) > w))] > (swap! local-res conj (f wu)))) > local-res))) > (range nthreads)) > results (doall (map #(deref (.get %)) ;; blocks until completion > (.invokeAll pool tasks))) ;; start all tasks > results (reduce concat results)] > (.shutdown pool) > ;; sanity check > (when-not (and (empty? @queue) > (= (count results) (count coll)) > (every? #(= % :done) results)) > (println "ERROR: queue " (count @queue) " #results" (count results))) > results)) > > Results on an i7 920, 4 cores/8 threads (hyperthreading), Ubuntu 10.10: > > user=> (time (last (map fast-or-slow inputs)))) > "Elapsed time: 161891.732036 msecs", 100% CPU (out of 800% possible) > > user=> (time (last (pmap fast-or-slow inputs)))) > "Elapsed time: 163139.249677 msecs", 100% CPU > pmap has zero effect on my system, it won't use more than one core. > > user=> (time (last (pmapall fast-or-slow inputs)))) > "Elapsed time: 37710.349712 msecs", ~793% CPU > > user=> (time (last (pmap-pool fast-or-slow inputs)))) > "Elapsed time: 36393.132824 msecs", ~795% CPU > > > -- > 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 > Note that posts from new members are moderated - please be patient with your > first post. > 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 -- Lee Spector, Professor of Computer Science Cognitive Science, Hampshire College 893 West Street, Amherst, MA 01002-3359 lspec...@hampshire.edu, http://hampshire.edu/lspector/ Phone: 413-559-5352, Fax: 413-559-5438 -- 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 Note that posts from new members are moderated - please be patient with your first post. 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
