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Locating the bottleneck

Thursday Mar 1st 2001

Three approaches to locating the performance-critical part of a program are:

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Three approaches to locating the performance-critical part of a program are:

1. Install your own instrumentation

“Profile” code by inserting explicit timing:

long start = System.currentTimeMillis();
   // Operation to be timed goes here
long time = System.currentTimeMillis() - start;

Have an infrequently-used method print cumulative times out to the console window with System.out.println( ). Since the compiler will ignore it when false, a static final boolean switch can turn the timing on and off so the code can efficiently be left in place in released code, ready for emergency use at any time. Even when more sophisticated profiling is available, this is a convenient way to time a specific task or operation.

System.currentTimeMillis( ) returns time in 1/1000ths of a second. However, some systems with time resolution less than a millisecond (such as a Windows PC) need to repeat an operation n times and divide the total time by n to get accurate estimates.

2. JDK profiling [2]

The JDK comes with a built-in profiler that keeps track of the time spent in each routine and writes the information to a file. Unfortunately, the JDK profilers have uneven performance. JDK 1.1.1 works, but subsequent releases have had various instabilities.

To run the profiler, use the -prof option when invoking the unoptimized versions of the Java interpreter, for example:

java_g -prof myClass

Or with an applet:

java_g -prof sun.applet.AppletViewer applet.html

The profiler output is not particularly easy to decipher. In fact, in JDK 1.0 it truncates the method names to 30 characters, so it might not be possible to distinguish between some methods. However, if your platform does support the -prof option, either Vladimir Bulatov’s HyperProf [3] or Greg White’s ProfileViewer [4] will help interpret the results.

3. Special tools

The best way to keep up with the exploding field of performance optimization tools is through a Web site such as Jonathan Hardwick’s Tools for Optimizing Java [5] at http://www.cs.cmu.edu/~jch/java/tools.html.

Tips for measuring performance

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