Web hosting packages - 362 CHAPTER 14 SOLVING COMMON JAVA EE
362 CHAPTER 14 SOLVING COMMON JAVA EE PERFORMANCE PROBLEMS Figure 14-6. The relationship between the permanent space and the heap In general you want the permanent space to be large enough to hold all classes in your application, because reading classes from the file system is obviously more expensive than reading them from memory. To help you ensure that classes are not unloaded from the permanent space, the JVM has a tuning option: noclassgc This option tells the JVM not to perform garbage collection on (and unload) the class files in the permanent space. This tuning option is very intelligent, but it raises a question: what does the JVM do if the permanent space is full when it needs to load a new class? In my observation, the JVM examines the permanent space and sees that it needs memory, so it triggers a major garbage collection. The garbage collection cleans up the heap, but cannot touch the permanent space, so its efforts are fruitless. The JVM then looks at the permanent space again, sees that it is full, and repeats the process again, and again, and again. When I first encountered this problem, the customer was complaining of very poor performance and an eventual OutOfMemoryError after a certain amount of time. After examining verbose garbage collection logs in conjunction with heap utilization and process memory utilization charts, I soon discovered that the heap was running well, but the process was running out of memory. This customer maintained literally thousands of JSPs, and as such each one was translated to Java code, compiled to bytecode, and loaded in the permanent space before
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