Hey, I'm trying to work on some performance sensitive code using Clojure
together with the Carrotsearch HPPC library. I've come up against a weird
behaviour of set! in conjunction with primitive maths.
This example is a toy problem not a production problem, but there are
things I might not be harder to do at work w/Clojure.
I have a com.carrotsearch.hppc.LongLongHashMap and I wish to sum the
contents of the map. They provide a com.carrotsearch.hppc.LongLongProcedure
where an apply method is called for each k, v.
Thence:
(defprotocol ValueRetriever
(get-value [this ^LongLongHashMap memory]))
(deftype ValueAdder [^{:unsynchronized-mutable true} ^long total]
LongLongProcedure
(^void apply [this ^long k ^long v]
(set! total (unchecked-add total v)))
ValueRetriever
(get-value [this memory] (set! total 0) (.forEach memory this) total))
To a first approximation all of the time spent summing the map is in the
apply method as expected, however when I profile it with YourKit every
sample taken is actually in clojure.lang.Numbers.num. Using the extremely
handy *clj-java-decompiler *library I can try to see what's happening, and
it looks like we're attempting to box the return value from set!
public void apply(final long k, final long n) {
Numbers.num(this.total += n);
}
Is there some technique I can use to stop the return value from set! being
boxed (before the box is discarded to the void)?
I do have real use cases where a rather tight aggregation loop will be
called for many millions of values and I'd prefer not to incur this cost.
Workaround is obviously to write the aggregators in Java but that's
strongly not preferred, at the point I'm mixing modes I might as well write
the whole core in Java.
Cheers,
Pete Windle
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