Java performance tips

Question

I have a program I ported from C to Java. Both apps use quicksort to order some partitioned data (genomic coordinates).

The Java version runs fast, but I'd like to get it closer to the C version. I am using the Sun JDK v6u14.

Obviously I can't get parity with the C application, but I'd like to learn what I can do to eke out as much performance as reasonably possible (within the limits of the environment).

What sorts of things can I do to test performance of different parts of the application, memory usage, etc.? What would I do, specifically?

Also, what tricks can I implement (in general) to change the properties and organization of my classes and variables, reducing memory usage and improving speed?

EDIT : I am using Eclipse and would obviously prefer free options for any third-party tools. Thanks!

Answer 1

Is your sorting code executing only once, e.g. in a commandline utility that just sorts, or multiple times, e.g. a webapp that sorts in response to some user input?

Chances are that performance would increase significantly after the code has been executed a few times because the HotSpot VM may optimize aggressively if it decides your code is a hotspot.

This is a big advantage compared to C/C++.

The VM, at runtime, optimizes code that is used often, and it does that quite well. Performance can actually rise beyond that of C/C++ because of this. Really. ;)

Your custom Comparator could be a place for optimization, though.

Try to check inexpensive stuff first (e.g. int comparison) before more expensive stuff (e.g. String comparison). I'm not sure if those tips apply because I don't know your Comparator.

Use either Collections.sort(list, comparator) or Arrays.sort(array, comparator). The array variant will be a bit faster, see the respective documentation.

As Andreas said before: don't try to outsmart the VM.

Answer 2

Profile and tune your java program and host machine. Most code follows 80/20 rule. That is 20% of code 80% of time, so find that 20% and make it as fast as possible. For example, the article Tuning Java Servers (http://www.infoq.com/articles/Tuning-Java-Servers) provides a description of drill down from command line and then isolate the problem using tools like Java Flight recorder, Eclipse Memory Analyser, and JProfiler.

Answer 3

jvisualvm ships with JDK 6 now - that's the reason the link cited above doesn't work. Just type "jvisualvm <pid>", where <pid> is the ID of the process you want to track. You'll get to see how the heap is being used, but you won't see what's filling it up.

If it's a long-running process, you can turn on the -server option when you run. There are a lot of tuning options available to you; that's just one.

Answer 4

Perhaps there are other routes to performance enhancement other than micro-optimization of code. How about a different algorithm to achieve what you wanted your program to do? May be a different data structure?

Or trade some disk/ram space for speed, or if you can give up some time upfront during the loading of your program, you can precompute lookup tables instead of doing calculations - that way, the processing is fast. I.e., make some trade-offs of other resources available.

Answer 5

Don't optimize prematurely.

Measure performance, then optimize.

Use final variables whenever possible. It will not only allow JVM to optimize more, but also make your code easier to read and maintain.

If you make your objects immutable, you don't have to clone them.

Optimize by changing the algorithm first, then by changing the implementation.

Sometimes you need to resort to old-style techniques, like loop unrolling or caching precalculated values. Remember about them, even if they don't look nice, they can be useful.

Answer 6

Also try tweaking the runtime arguments of the VM - the latest release of the VM for example includes the following flag which can improve performance in certain scenarios.

-XX:+DoEscapeAnalysis