Shared memory segments reside in real memory (we hope), and a process creates them using the shmget call. Most DBAs quickly figure this out once they try to start an Oracle instance and receive a shmget-related message, and then need to track down what the heck just happened.

What is unique about shared memory segments is that multiple processes can access the same piece of memory. The alternative would be to place the common data on disk. What a locking nightmare that would be! On Linux and Unix systems, Oracle's SGA is stored in shared memory segments.

Process-related memory can commonly be divided into three areas: code, data, and stack. Different operating systems will give these different names, and will also have different and additional process-related memory categories. AIX in particular has its own way of categorizing process memory. If you need to know the actual memory a process is consuming, use an operating system-specific command. For example, both Solaris and Linux provide the pmap command to get process-level memory details. I'll quickly describe each of the common categories: