Microbenchmark on Java and C Matrix Multiplication

My Java bug entry on faster numerics has been summarily closed as unreproducible. There is a short evaluation showing Java to be faster that C at matrix multiplication, but the comparison misses the mark because:

  • The C code (not shown) appears to be using nested arrays for matrix storage, which I have never seen in practice for C matrix libraries.
  • The Java code is run under the server version of Java, which is fine for a small benchmark but usually not practical for a large (client) app, such as many existing C++ numerics apps are.

Nonetheless, the activity prompted me to rerun my matrix benchmarks. Actually, I had to recreate them since I couldn’t find my Java 1.3 era versions that showed a 15x disadvantage for Java. This time Java fared much better. The tests (C++ Code Java Code) just multiply the same 200×200 matrices over and over with some add operations mixed in for good measure. I tried to make the code fast without working too hard. For instance, I didn’t get GCC to do vectorization, and I didn’t control my environment too well.

I tried two variations of the Java matrix representation and two variations of the Java multiplication code for four total Java variations, of which I took the fastest. The representation variation was whether to use a single array and multiplied indexing or to use nested arrays. The multiply variation was whether to transpose the B matrix or not — transposing it lets the inner loop traverse a one sub-array each from A and B. Arrays of arrays with transposing worked best on my Intel Mac, but flat arrays without transposing was best on the PowerPC Mac. I think the Intel JVM is better about the range check elimination optimization, so it favors the extra arrays with simpler indexing.

2GHz PowerPC Mac

Java 1.5 56.7ms
C++/GCC 26.2 ms

2.33GHz Intel MacBook

Java 1.5/client 32.5 ms
Java 1.5/server 12.6 ms
C++/GCC 11.0 ms

On the PowerPC Mac, there was no difference between client and server Java.

So client Java 1.5 is only 3x of C++ and server Java 1.5 is almost the same as C++. Not bad, but not quite what Sun’s evaluation showed. Still need to try Java 6 and turning on GCC vector processing.

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