There have been at least two bugs in the ColdFusion MX Administrator's ability to properly update the JVM settings in cf_root/runtime/bin/jvm.config when using the Java/JVM page to enact the change. A technote was just released for How to upgrade the JVM shipped with ColdFusion, which demonstrates how to change JVM settings from the Administrator. I recommend that you familiarize yourself with the contents of the jvm.config file and carefully edit the file by hand instead, after making an original back up copy.

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