Java running faster than C

Note: A lot of people seem to be taking this post to be the "Ultimate C vs Java shootout". It's not. Performance is a very complex topic. My only real point is this: Java (which used to be slow) has reached the class of "fast languages". For the majority of applications, speed is no longer a valid excuse for using C++ instead of Java.

I just saw this page comparing the performance of several languages on a simple Mandelbrot set generator. His numbers show Java being over twice as slow as C, but then I noticed that he's using an older version of java and only running the test once, which doesn't really give the JVM a chance to show off.

I quickly hacked up the code to run 100 iterations 3 times and then used my standard "go fast" flags (there may be better flags, but I'm lazy). Here are my results:

$ java -server -XX:CompileThreshold=1 Mandelbrot 2>/dev/null
Java Elapsed 2.994
Java Elapsed 1.926
Java Elapsed 1.955

$ gcc -O8 mandelbrot.c
$ ./a.out 2>/dev/null
C Elapsed 2.03
C Elapsed 2.04
C Elapsed 2.05

C still wins on the first iteration, but Java is actually slightly faster on subsequent iterations!

Obviously the results will be different with different code and different machines, but it's clear that the JVM is getting quite fast.

This test was run with Java 1.6.0-b105 and gcc 4.1.2 under Linux 2.6.17 under Parallels on my 2.33GHz Core 2 Duo MacBook Pro. Here is the hacked up code: Java and C.


For extra fun, I also tried running the JS test using the Rhino compiler:

$ java -cp rhino1_6R5/js.jar -server -XX:CompileThreshold=1 org.mozilla.javascript.tools.shell.Main -O 9 mandelbrot.js 2>/dev/null
JavaScript Elapsed 21.95
JavaScript Elapsed 17.039
JavaScript Elapsed 17.466
JavaScript Elapsed 17.147

Compiled JS is about 9x slower than C on this test. If CPU speed doubles every 18 months, then JS in 2007 performs like C in 2002.


Update: A few more cpp flags have been suggested. -march=pentium4 helps a little, but it's still slower than Java.

$ gcc -O9 -march=pentium4 mandelbrot2.c
$ ./a.out 2>/dev/null
C Elapsed 1.99
C Elapsed 1.99
C Elapsed 1.99

Adding -ffast-math puts C in the lead, but I'm not sure what the downside is. The gcc man page says, "This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions." That sounds like an optimization that Java might not use.

$ gcc -ffast-math -O9 -march=pentium4 mandelbrot2.c
$ ./a.out 2>/dev/null
C Elapsed 1.66
C Elapsed 1.67
C Elapsed 1.67

Update: Several people have claimed that the performance difference is due to fputs (including the top rated comment on reddit, aumusingly). That is not correct, at least not on my computer. I tried replacing the print calls with a trivial function (but with side-effects), and it actually helped the Java more than the C:

C Elapsed 1.88
Java Elapsed 1.554

Many people have pointed out that '-O8' is more than enough 'O' levels. I know, and I don't care -- it's just as good as '-O3' or whatever.