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The main disadvantage of using the VM system is that, if memory requirements significantly exceed the available RAM, system performance will degrade. As application memory requirements start exceeding the amount of physical memory available in the system, paging to disk will begin. This causes rapid performance degradation because, as you saw in the previous section, disk access is many orders of magnitude slower than memory access of any kind.
Eventually, if the application requires much more memory than the available physical memory, so-called "thrashing" may occur, which makes the system practically impossible to use. For detailed descriptions of the Solaris VM system and other system-related information, see Sun Performance and Tuning: Java and the Internet by Adrian Cockcroft and Richard Pettit (Prentice Hall) and Solaris Internals: Architecture, Tips and Techniques, Volume 1: Core Kernel by Richard McDougall and Jim Mauro (to be published by Prentice Hall). McDougall and Mauro's columns are also available online at SunWorld (http://www.sunworld.com).
Pro/ENGINEER is a large application with the following features (from the memory efficiency perspective):
· The use of the malloc() interface to obtain all required memory from the system. Most applications use this method, although other methods exist (mmap-based for example). C++ operator "new" also belongs to this category since most implementations of "new" call malloc() internally.
· Highly dynamic memory usage. In other words, the amount of memory a Pro/ENGINEER session requires is highly dependent upon the size of the model and the operations performed on it.
· Large amounts of memory required for large models. Pro/ENGINEER memory consumption can vary from about 50 megabytes to a number of gigabytes, up to the total amount of swap space (RAM plus disk swap) available in the system.
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