Java Performance Tuning

What to Measure

The main measurement is always wall-clock time. You should use this measurement to specify almost all benchmarks, as it’s the real-time interval that is most appreciated by the user. (There are certain situations, however, in which system throughput might be considered more important than the wall-clock time; e.g., servers, enterprise transaction systems, and batch or background systems.)

The obvious way to measure wall-clock time is to get a timestamp using System.currentTimeMillis() and then subtract this from a later timestamp to determine the elapsed time. This works well for elapsed time measurements that are not short.^[5] Other types of measurements have to be system-specific and often application-specific. You can measure:

• CPU time (the time allocated on the CPU for a particular procedure)

• The number of runnable processes waiting for the CPU (this gives you an idea of CPU contention)

• Paging of processes

• Memory sizes

• Disk throughput

• Disk scanning times

• Network traffic, throughput, and latency

• Transaction rates

• Other system values

However, Java doesn’t provide mechanisms for measuring these values directly, and measuring them requires at least some system knowledge, and usually some application-specific knowledge (e.g., what is a transaction for your application?).

Note

You need to be careful when running tests that have small differences in timings. The first test is usually slightly slower than any other tests. Try doubling the test run so that each test is run twice ...