1.8GB Heap Limit in ColdFusion MX

June 08, 2004

Macromedia's Recent Technote: Maximum JVM heap size greater than 1.8GB will prevent ColdFusion MX from starting points out the limitations of a the JVM on a 32 bit architecture.

Wondering where the number 1.8GB comes from? Moazam Raja's recent blog entry explains. 32 bit processes can only address 4GB of memory, windows reserves 2GB for the kernel. So where's the other .2 GB? It is allocated to the the JVM's permanent generation (this stores meta data for the JVM). Many people increase the size of the permanent generation (for CFM or JSP apps) using the jvm startup option -XX:MaxPermSize. If you increase the perm generation (and you may want to) keep in mind your Max heap size will be less than 1.8GB.

Of course if you have these problems you should consider upgrading to a 64 bit architecture which have much larger limits.

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Pete, as usual this is great information. Thanks! Your research skills amaze me.
As far as 32-bit OSes go, Mac OS X is actually somewhat ahead in the memory game. The virtual address space available to an application is close to 4GB, which is more than the 2GB Windows provides and the 3GB Linux provides. And the Mac's contiguous address space of 2.25GB exceeds the 1.5-1.7GB on Windows and the 2GB for Linux. Furthermore, Apple will not require you to shell out big money for some "Advanced Server" version of its OS if you want to install more than 4GB of physical memory in your machine, which is what Microsoft and Red Hat want you to do.
Fedora Core 2 has the 4G/4G patch, so userspace apps get 4GB virtual address space.

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