a quick performance comparison of java on raspbian

Oracle released a preview with armhf support.
http://jdk8.java.net/fxarmpreview/

This gives us a boost.

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pi@raspberrypi ~/java/testprime/bin $ time java testprime > /dev/null

real   0m5.505s
user   0m4.620s
sys   0m0.170s


As Henrik described above, using std out is a very expensive operation and you're not quite comparing raw cpu power. If you were to remove all calls to printf and compile with -O2, you would get the following

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0.00s user 0.00s system 0% cpu 0.009 total

Did you check the assembly that the optimizer does not just skip all the code?

Maybe something like this would be better (not sure how easy/hard the bit ops are in other than C or JAVA though, if want to make it a really portable test):

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#include  <stdio.h>
int result = 0;
void main() {
    result |= 2;
    for (int i=3;i<=10000;i++) {
      int prime = 1;
      for (int j=2;j<i;j++) {
        int k = i/j;
        int l = k*j;
        if (l==i) prime = 0;
      }
      if (prime) result |= i;
    }
    printf("%i\n", result);
}


And to give the Hotspot compiler to chance to do the magic (maybe) something like this (feel free to comment if not enough to activate it):

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public class Prime3 {

  public static void main(String [] args) {
   for (int i = 0; i < 10; i++) {
      test();
   }
  }
 
  public static void test() {
    int result = 0;
    result |= 2;
    for (int i=3;i<=10000;i++) {
      boolean prime = true;
      for (int j=2;j<i;j++) {
        int k = i/j;
        int l = k*j;
        if (l==i) prime = false;
      }
      if (prime) result |= i;
    }
   System.out.println(result);
  }
}


and the same in C

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#include  <stdio.h>
int test() {
   int result = 0;
    result |= 2;
    for (int i=3;i<=10000;i++) {
      int prime = 1;
      for (int j=2;j<i;j++) {
        int k = i/j;
        int l = k*j;
        if (l==i) prime = 0;
      }
      if (prime) result |= i;
    }
    printf("%i\n", result);
}

int main() {
   for (int i = 0; i < 10; i++) {
      test();
   }
}


Interesting results running under Cygwin on my Win7 laptop:

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*** C ***
gcc (GCC) 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)

--- print
real    0m2.835s !!??
user    0m0.358s
sys     0m0.046s

--- or
real    0m0.441s
user    0m0.374s
sys     0m0.030s

--- 10 * or
real    0m3.691s
user    0m3.556s
sys     0m0.030s

*** Java ***
java version "1.6.0_37"
Java(TM) SE Runtime Environment (build 1.6.0_37-b06)
Java HotSpot(TM) Client VM (build 20.12-b01, mixed mode, sharing)

--- print
real    0m0.545s
user    0m0.015s
sys     0m0.030s

--- or
real    0m0.479s
user    0m0.000s
sys     0m0.031s

--- 10 * or
real    0m3.521s
user    0m0.015s
sys     0m0.015s


...the printf in C under Cygwin must be doing something very odd

I've also made my own bench using matrix computation to see float performance
You may take on look on http://www.raspberrypi.org/phpBB3/viewt ... 31&t=29421
All detail on http://trouch.com/2013/01/12/raspberry-pi-benchmark/

Hi, this is not related to loop unrolling which is a specific optimization in the JIT. The general issue you see is that the method can be optimized but the optimized method will not be used until the next invocation. It is technically possible to switch out a method in flight. This is called on stack replacement but is quite complex and is mostly avoided since it provides limited benefit and is expensive to maintain. Plenty of good literature out there. My favorite is the Oracle JRockit book, which is a deep dive into how JVMs work (different JVM, same principles, and Hotspot and JRockit are being merged by us so valuable insights anyway). You can find it on Amazon.

There are some reasonable benchmarks out there. Try SPECjvm2008, it is fairly comprehensive.

For a good microbenchmark, try this:

main
while (true)
read time
call benchmark method
read time, print out difference and result
end while

benchmark method
run benchmark, maybe lots of times
store intermediary results in a public variable to avoid the optimizer removing the code you want to measure
return result

Hi Henrik, (I'm guessing it's Henrik Stahl, right?) how's life since the Oracle takeover?

Does the new merged Jrockit/Hotspot implementation generate ARM optimised bytecode - so, for example does it know about all the conditional branch instructions? I could imagine that in a synthetic benchmark like this, that a loop unrolling vs conditional brach decision might make a big difference over the lifetime of this loop.

Martin

PS Guys (and gals), Henrik was one of the folks who built the JRockit JVM so he's a goldmine for this kind of stuff....sorry Henrik, stitched you up there

Hi, this is not related to loop unrolling which is a specific optimization in the JIT. The general issue you see is that the method can be optimized but the optimized method will not be used until the next invocation. It is technically possible to switch out a method in flight. This is called on stack replacement but is quite complex and is mostly avoided since it provides limited benefit and is expensive to maintain. Plenty of good literature out there. My favorite is the Oracle JRockit book, which is a deep dive into how JVMs work (different JVM, same principles, and Hotspot and JRockit are being merged by us so valuable insights anyway). You can find it on Amazon.

There are some reasonable benchmarks out there. Try SPECjvm2008, it is fairly comprehensive.

For a good microbenchmark, try this:

main
while (true)
read time
call benchmark method
read time, print out difference and result
end while

benchmark method
run benchmark, maybe lots of times
store intermediary results in a public variable to avoid the optimizer removing the code you want to measure
return result

Hi Henrik, (I'm guessing it's Henrik Stahl, right?) how's life since the Oracle takeover?

Does the new merged Jrockit/Hotspot implementation generate ARM optimised bytecode - so, for example does it know about all the conditional branch instructions? I could imagine that in a synthetic benchmark like this, that a loop unrolling vs conditional brach decision might make a big difference over the lifetime of this loop.

Martin

PS Guys (and gals), Henrik was one of the folks who built the JRockit JVM so he's a goldmine for this kind of stuff....sorry Henrik, stitched you up there

Hi Martin,

Yes, same Henrik. Thanks for the kind words! The Oracle JVM is based on Hotspot with features from JRockit ported over. The port to Linux/ARM includes a JIT is highly optimized for ARM, which means it does many of the optimizations you would find in a textbook on the subject and then some. I'm not the least surprised that people report results similar to and in some cases exceeding gcc - if you were to look at the generated assembly code you would see that it is much the same. A JVM has some benefits over a static compiler in that it knows the performance characteristics of the current executing program when it performs the compilation, so it can do some tricks such as adjusting the memory layout to avoid CPU cache misses etc. On the other hand, it has some drawbacks such as slower startup and JNI overhead for calling native libraries. Net-net for a non-trivial program the performance tends to be approximately the same as for a normal C program compiled with gcc. But for microbenchmarks C tends to perform better (unless the Java coder takes care to avoid common pitfalls such as the lack of on stack replacement).

Given the maturity of our Java port to ARM and the Java toolchain, and the speed advantage over Python it would IMHO be good for everyone if Java had a more prominent place in the RPi community.

Henrik

Given the maturity of our Java port to ARM and the Java toolchain, and the speed advantage over Python it would IMHO be good for everyone if Java had a more prominent place in the RPi community.

I totally agree !

But even if I am a C/Java developer prior to Python, this one has some advantages for a totally novice : interpreter mode, no compilation required, object oriented coding is optional, and ease to do many things. I may say Python should be followed by Java from a learning pov if syntax was not so different.
So people should directly start with Java, but I'm not sure the Pi will handle a full Eclipse or Netbeans. It requires another computer but there is others issues, that can be solved, but at the end, the setup will be more complex for a novice.

Choosing a first programming language to learn is still not easy...

But for intermediate and advanced developers, Java is definitively a very nice choice for the Pi.

I'm happy to hear that there are developments being made with Java on the R-Pi platform. Something that I'm very interested in is running Processing on the R-Pi. I think it's a wonderful educational environment, and if it can perform well it would be ideal for kids etc. looking for an easy introduction into software development.