Java Performance Tuning

CPU

Java provides a virtual machine runtime system that is just that: an abstraction of a CPU that runs in software. These virtual machines run on a real CPU, and in this section I discuss the performance characteristics of those real CPUs.

CPU Load

The CPU and many other parts of the system can be monitored using system-level utilities. On Windows, the task manager and performance monitor can be used for monitoring. On Unix, a performance monitor (such as perfmeter) is usually available, as well as utilities such as vmstat. Two aspects of the CPU are worth watching as primary performance points. These are the CPU utilization (usually expressed in percentage terms) and the runnable queue of processes and threads (often called the load or the task queue). The first indicator is simply the percentage of the CPU (or CPUs) being used by all the various threads. If this is up to 100% for significant periods of time, you may have a problem. On the other hand, if it isn’t, the CPU is underutilized, but that is usually preferable. Low CPU usage can indicate that your application may be blocked for significant periods on disk or network I/O. High CPU usage can indicate thrashing (lack of RAM) or CPU contention (indicating that you need to tune the code and reduce the number of instructions being processed to reduce the impact on the CPU).

A reasonable target is 75% CPU utilization. This means that the system is being worked towards its optimum, but that you have left some slack for ...