Here’s a great overview of the JVM memory model (for all of Sun’s JVMs, including the latest changes). I find it intensely interesting that he brushes over what, for me, is the most complicated part of any web application tuning–testing. He outlines a process for initially sizing the various compartments of the JVM heap (for versions 1.4 and below), and then says: ‘The [resizing] process continues by “testing and tweaking” until things look good.’

Wow. Talk about waving your hands. I did some testing of a web application a few months ago, but when I presented the results, I was very clear that they were guidelines only. I didn’t have the resources or ingenuity to replicate the behavior of real users on real clients. A few years ago, I was part of a project that ran aground on this same rock, costing the company I was working for plenty of money. Using software to imitate user behavior is hard. A short list of the differences between software and users:

1. Users get distracted–by popups, their kids, etc. Software, not so much.
2. Users are connecting via a variety of methods, with a wide range of quality levels–modems, broadband.
3. Users don’t use applications in the way developers intended. The testing software, on the other hand, is programmed by developers, who naturally have it use the application in the way they intended

The adaptive memory model for the next JDK (wow, now it’s J2SE 5–Sun pulled another Solaris reversioning trick) that the author outlines might make the “tweaking” portion of his hand waving, err, I mean tuning, easier but leaves the “testing” as difficult as ever.

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