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Java theory and practice: Garbage collection and performance
By Brian Goetz - 2004-03-03 Page:  1 2 3 4 5 6

When good performance advice goes bad

A cover story in the July 2003 Java Developer's Journal illustrates how easy it is for good performance advice to become bad performance advice by simply failing to adequately identify the conditions under which the advice should be applied or the problem it was intended to solve. While the article contains some useful analysis, it will likely do more harm than good (and, unfortunately, far too much performance-oriented advice falls into this same trap).

The article opens with a set of requirements from a realtime environment, where unpredictable garbage collection pauses are unacceptable and there are strict operational requirements on how long a pause can be tolerated. The authors then recommend nulling references, object pooling, and scheduling explicit garbage collection to meet the performance goals. So far, so good -- they had a problem and they figured out what they had to do to solve it (although they appear to have failed to identify what the costs of these practices were or explore some less intrusive alternatives, such as concurrent collection). Unfortunately, the article's title ("Avoid Bothersome Garbage Collection Pauses") and presentation suggest that this advice would be useful for a wide range of applications -- perhaps all Java applications. This is terrible, dangerous performance advice!

For most applications, explicit nulling, object pooling, and explicit garbage collection will harm the throughput of your application, not improve it -- not to mention the intrusiveness of these techniques on your program design. In certain situations, it may be acceptable to trade throughput for predictability -- such as real-time or embedded applications. But for many Java applications, including most server-side applications, you probably would rather have the throughput.

The moral of the story is that performance advice is highly situational (and has a short shelf life). Performance advice is by definition reactive -- it is designed to address a particular problem that occurred in a particular set of circumstances. If the underlying circumstances change, or they are simply not applicable to your situation, the advice may not be applicable, either. Before you muck up your program's design to improve its performance, first make sure you have a performance problem and that following the advice will solve that problem.



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First published by IBM developerWorks


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