What about a more direct way of creating your threads. This code is
too simple and more is needed to collect results with futures, but I
wonder how something like this would perform on your machine:
(defn burn-via-pool [n]
(print n " burns via a thread pool: ")
(time
(let [cores (.. Runtime getRuntime availableProcessors)
pool (java.util.concurrent.Executors/newFixedThreadPool
cores)
^Callable func (fn [] (burn))]
(dotimes [_ n] (.submit pool func))
(.shutdown pool)
(.awaitTermination pool 1 java.util.concurrent.TimeUnit/
HOURS))))
On Aug 4, 7:36 am, Lee Spector <[email protected]> wrote:
> Apologies for the length of this message -- I'm hoping to be complete, but
> that made the message pretty long.
>
> Also BTW most of the tests below were run using Clojure 1.1. If part of the
> answer to my questions is "use 1.2" then I'll upgrade ASAP (but I haven't
> done so yet because I'd prefer to be confused by one thing at a time :-). I
> don't think that can be the full answer, though, since the last batch of runs
> below WERE run under 1.2 and they're also problematic...
>
> Also, for most of the runs described here (with the one exception noted
> below) I am running under Linux:
>
> [lspec...@fly ~]$ cat /proc/version
> Linux version 2.6.18-164.6.1.el5 ([email protected]) (gcc
> version 4.1.2 20080704 (Red Hat 4.1.2-46)) #1 SMP Tue Nov 3 16:12:36 EST 2009
>
> with this Java version:
>
> [lspec...@fly ~]$ java -version
> java version "1.6.0_16"
> Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
> Java HotSpot(TM) 64-Bit Server VM (build 14.2-b01, mixed mode)
>
> SO: Most of the documentation and discussion about clojure concurrency is
> about managing state that may be shared between concurrent processes, but I
> have what I guess are more basic questions about how concurrent processes
> can/should be started even in the absence of shared state (or when all that's
> shared is immutable) and about how to get the most out of concurrency on
> multiple cores.
>
> I often have large numbers of relatively long, independent processes and I
> want to farm them out to multiple cores. (For those who care this is often in
> the context of evolutionary computation systems, with each of the processes
> being a fitness test.) I had thought that I was farming these out in the
> right way to multiple cores, using agents or sometimes just pmap, but then I
> noticed that my runtimes weren't scaling in the way that I expected across
> machines with different numbers of cores (even though I usually saw near
> total utilization of all cores in "top").
>
> This led me to do some more systematic testing and I'm confused/concerned
> about what I'm seeing, so I'm going to present my tests and results here in
> the hope that someone can clear things up for me. I know that timing things
> in clojure can be complicated both on account of laziness and on account of
> optimizations that happen on the Java side, but I think I've done the right
> things to avoid getting tripped up too much by these issues. Still, it's
> quite possible that I've coded some things incorrectly and/or that I'm
> misunderstanding some basic concepts, and I'd appreciate any help that anyone
> can provide.
>
> First I defined a function that would take a non-trivial amount of time to
> execute, as follows:
>
> (defn burn
> ([] (count
> (take 1E6
> (repeatedly
> #(* 9999999999 9999999999)))))
> ([_] (burn)))
>
> The implementation with an ignored argument just serves to make some of my
> later calls neater -- I suppose I might incur a tiny additional cost when
> calling it that way but this will be swamped by the things I'm timing.
>
> Then I defined functions for calling this multiple times either sequentially
> or concurrently, using three different techniques for starting the concurrent
> processes:
>
> (defn burn-sequentially [n]
> (print n " sequential burns: ")
> (time (dotimes [i n] (burn))))
>
> (defn burn-via-pmap [n]
> (print n " burns via pmap: ")
> (time (doall (pmap burn (range n)))))
>
> (defn burn-via-futures [n]
> (print n " burns via futures: ")
> (time (doall (pmap deref (map (fn [_] (future (burn)))
> (range n))))))
>
> (defn burn-via-agents [n]
> (print n " burns via agents: ")
> (time (let [agents (map #(agent %) (range n))]
> (dorun (map #(send % burn) agents))
> (apply await agents))))
>
> Finally, since there's often quite a bit of variability in the run time of
> these things (maybe because of garbage collection? Optimization? I'm not
> sure), I define a simple macro to execute a call three times:
>
> (defmacro thrice [expression]
> `(do ~expression ~expression ~expression))
>
> Now I can do some timings, and I'll first show you what happens in one of the
> cases where everything performs as expected.
>
> On a 16-core machine (details
> athttp://fly.hampshire.edu/ganglia/?p=2&c=Rocks-Cluster&h=compute-4-1.l...),
> running four burns thrice, with the code:
>
> (thrice (burn-sequentially 4))
> (thrice (burn-via-pmap 4))
> (thrice (burn-via-futures 4))
> (thrice (burn-via-agents 4))
>
> I get:
>
> 4 sequential burns: "Elapsed time: 2308.616 msecs"
> 4 sequential burns: "Elapsed time: 1510.207 msecs"
> 4 sequential burns: "Elapsed time: 1182.743 msecs"
> 4 burns via pmap: "Elapsed time: 470.988 msecs"
> 4 burns via pmap: "Elapsed time: 457.015 msecs"
> 4 burns via pmap: "Elapsed time: 446.84 msecs"
> 4 burns via futures: "Elapsed time: 417.368 msecs"
> 4 burns via futures: "Elapsed time: 401.444 msecs"
> 4 burns via futures: "Elapsed time: 398.786 msecs"
> 4 burns via agents: "Elapsed time: 421.103 msecs"
> 4 burns via agents: "Elapsed time: 426.775 msecs"
> 4 burns via agents: "Elapsed time: 408.416 msecs"
>
> The improvement from the first line to the second is something I always see
> (along with frequent improvements across the three calls in a "thrice"), and
> I assume this is due to optimizations talking place in the JVM. Then we see
> that all of the ways of starting concurrent burns perform about the same, and
> all produce a speedup over the sequential burns of somewhere in the
> neighborhood of 3x-4x. Pretty much exactly what I would expect and want. So
> far so good.
>
> However, in the same JVM launch I then went on to do the same thing but with
> 16 and then 48 burns in each call:
>
> (thrice (burn-sequentially 16))
> (thrice (burn-via-pmap 16))
> (thrice (burn-via-futures 16))
> (thrice (burn-via-agents 16))
>
> (thrice (burn-sequentially 48))
> (thrice (burn-via-pmap 48))
> (thrice (burn-via-futures 48))
> (thrice (burn-via-agents 48))
>
> This produced:
>
> 16 sequential burns: "Elapsed time: 5821.574 msecs"
> 16 sequential burns: "Elapsed time: 6580.684 msecs"
> 16 sequential burns: "Elapsed time: 6648.013 msecs"
> 16 burns via pmap: "Elapsed time: 5953.194 msecs"
> 16 burns via pmap: "Elapsed time: 7517.196 msecs"
> 16 burns via pmap: "Elapsed time: 7380.047 msecs"
> 16 burns via futures: "Elapsed time: 1168.827 msecs"
> 16 burns via futures: "Elapsed time: 1068.98 msecs"
> 16 burns via futures: "Elapsed time: 1048.745 msecs"
> 16 burns via agents: "Elapsed time: 1041.05 msecs"
> 16 burns via agents: "Elapsed time: 1030.712 msecs"
> 16 burns via agents: "Elapsed time: 1041.139 msecs"
> 48 sequential burns: "Elapsed time: 15909.333 msecs"
> 48 sequential burns: "Elapsed time: 14825.631 msecs"
> 48 sequential burns: "Elapsed time: 15232.646 msecs"
> 48 burns via pmap: "Elapsed time: 13586.897 msecs"
> 48 burns via pmap: "Elapsed time: 3106.56 msecs"
> 48 burns via pmap: "Elapsed time: 3041.272 msecs"
> 48 burns via futures: "Elapsed time: 2968.991 msecs"
> 48 burns via futures: "Elapsed time: 2895.506 msecs"
> 48 burns via futures: "Elapsed time: 2818.724 msecs"
> 48 burns via agents: "Elapsed time: 2802.906 msecs"
> 48 burns via agents: "Elapsed time: 2754.364 msecs"
> 48 burns via agents: "Elapsed time: 2743.038 msecs"
>
> Looking first at the 16-burn runs, we see that concurrency via pmap is
> actually generally WORSE than sequential. I cannot understand why this should
> be the case. I guess if I were running on a single core I would expect to see
> a slight loss when going to pmap because there would be some cost for
> managing the 16 threads that wouldn't be compensated for by actual
> concurrency. But I'm running on 16 cores and I should be getting a major
> speedup, not a slowdown. There are only 16 threads, so there shouldn't be a
> lot of time lost to overhead.
>
> Also interesting, in this case when I start the processes using futures or
> agents I DO see a speedup. It's on the order of 6x-7x, not close to the 16x
> that I would hope for, but at least it's a speedup. Why is this so different
> from the case with pmap? (Recall that my pmap-based method DID produce about
> the same speedup as my other methods when doing only 4 burns.)
>
> For the calls with 48 burns we again see nearly the expected, reasonably good
> pattern with all concurrent calls performing nearly equivalently (I suppose
> that the steady improvement over all of the calls is again some kind of JVM
> optimization), with a speedup in the concurrent calls over the sequential
> calls in the neighborhood of 5x-6x. Again, not the ~16x that I might hope
> for, but at least it's in the right direction. The very first of the pmap
> calls with 48 burns is an anomaly, with only a slight improvement over the
> sequential calls, so I suppose that's another small mystery.
>
> The big mystery so far, however, is in the case of the 16 burns via pmap,
> which is bizarrely slow on this 16-core machine.
>
> Next I tried the same thing on a 48 core machine
> (http://fly.hampshire.edu/ganglia/?p=2&c=Rocks-Cluster&h=compute-4-2.l...).
> Here I got:
>
> 4 sequential burns: "Elapsed time: 3062.871 msecs"
> 4 sequential burns: "Elapsed time: 2249.048 msecs"
> 4 sequential burns: "Elapsed time: 2417.677 msecs"
> 4 burns via pmap: "Elapsed time: 705.968 msecs"
> 4 burns via pmap: "Elapsed time: 679.865 msecs"
> 4 burns via pmap: "Elapsed time: 685.017 msecs"
> 4 burns via futures: "Elapsed time: 687.097 msecs"
> 4 burns via futures: "Elapsed time: 636.543 msecs"
> 4 burns via futures: "Elapsed time: 660.116 msecs"
> 4 burns via agents: "Elapsed time: 708.163 msecs"
> 4 burns via agents: "Elapsed time: 709.433 msecs"
> 4 burns via agents: "Elapsed time: 713.536 msecs"
> 16 sequential burns: "Elapsed time: 8065.446 msecs"
> 16 sequential burns: "Elapsed time: 8069.239 msecs"
> 16 sequential burns: "Elapsed time: 8102.791 msecs"
> 16 burns via pmap: "Elapsed time: 11288.757 msecs"
> 16 burns via pmap: "Elapsed time: 12182.506 msecs"
> 16 burns via pmap: "Elapsed time: 14609.397 msecs"
> 16 burns via futures: "Elapsed time: 2519.603 msecs"
> 16 burns via futures: "Elapsed time: 2436.699 msecs"
> 16 burns via futures: "Elapsed time: 2776.869 msecs"
> 16 burns via agents: "Elapsed time:
> ...
>
> read more »
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