Marshall:
I'm not practiced in recognizing megamorphic call sites, so I could be missing
some in the example code below, modified from Lee's original code. It doesn't
use reverse or conj, and as far as I can tell doesn't use PersistentList,
either, only Cons.
(defn burn-cons [size]
(let [size (long size)]
(loop [i (long 0)
value nil]
(if (>= i size)
(last value)
(recur (inc i) (clojure.lang.Cons.
(* (int i)
(+ (float i)
(- (int i)
(/ (float i)
(inc (int i))))))
value))))))
(a) invoke (burn-cons 2000000) sequentially 64 times in a single JVM
(b) invoke (burn-cons 2000000) 64 times using a modified version of pmap that
limits the number of active threads to 2 (see below), in a single JVM. I would
hope that this would take about half the elapsed time than (a), but the elapsed
time is longer than (a)
(c) start up two JVMs simultaneously and invoke (burn-cons 2000000)
sequentially 32 times in each. The elapsed time here is less than (a), as I
would expect.
(Clojure 1.4, Oracle/Apple JDK 1.6.0_37, Mac OS X 10.6.8, running on a machine
with Intel core i7 with 4 physical cores but OS X reports it as 8 I think
because of 2 hyperthreads per core -- more details available on request).
Can you try to reproduce to see if you get similar results? If so, do you know
why we get bad parallelism in a single JVM for this code? If there are no
megamorphic call sites, then it is examples like this that lead me to wonder
about locking in memory allocation and/or GC.
With the functions below, my part (b) was measured by doing:
(time (doall (nthreads-pmap 2 (burn-cons 2000000) (unchunk (range 64)))))
Andy
(defn unchunk [s]
(when (seq s)
(lazy-seq
(cons (first s)
(unchunk (next s))))))
(defn nthreads-pmap
"Like pmap, except can take an argument nthreads to control the
maximum number of parallel threads used."
([f coll]
(let [n (+ 2 (.. Runtime getRuntime availableProcessors))]
(nthreads-pmap n f coll)))
([nthreads f coll]
(if (= nthreads 1)
(map f coll)
(let [n (dec nthreads)
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)))))
([nthreads f coll & colls]
(let [step (fn step [cs]
(lazy-seq
(let [ss (map seq cs)]
(when (every? identity ss)
(cons (map first ss) (step (map rest ss)))))))]
(nthreads-pmap nthreads #(apply f %) (step (cons coll colls))))))
On Dec 11, 2012, at 10:06 AM, Marshall Bockrath-Vandegrift wrote:
> Lee Spector <lspec...@hampshire.edu> writes:
>
>> If the application does lots of "list processing" but does so with a
>> mix of Clojure list and sequence manipulation functions, then one
>> would have to write private, list/cons-only versions of all of these
>> things? That is -- overstating it a bit, to be sure, but perhaps not
>> entirely unfairly -- re-implement Clojure's Lisp?
>
> I just did a quick look over clojure/core.clj, and `reverse` is the only
> function which stood out to me as hitting the most pathological case.
> Every other `conj` loop over a user-provided datastructure is `conj`ing
> into an explicit non-list/`Cons` type.
>
> So I think if you replace your calls to `reverse` and any `conj` loops
> you have in your own code, you should see a perfectly reasonable
> speedup.
>
> -Marshall
>
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