Locating the bottleneck

Bruce Eckel’s Thinking in Java Contents | Prev | Next

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

3.
Special tools

Tips
for measuring performance

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