Memory and System Performance

Performance suffers when the programs running on the system require more physical memory than is available. When this happens, the operating system begins paging and swapping, which is costly in both disk and CPU overhead.

Paging involves moving pages that have not been recently referenced to a free list of available memory pages. Most of the kernel resides in main memory and is not pageable.

Swapping occurs if the page daemon cannot keep up with the demand for memory. The swapper will attempt to swap out sleeping or stopped lightweight processes (LWPs). If there are no sleeping or stopped LWPs, the swapper will swap out a runnable process. The swapper will swap LWPs back in based on their priority. It will attempt to swap in processes that are runnable.

Swap Space

Swap areas are really file systems used for swapping. Swap areas should be sized based on the requirements of your applications. Check with your vendor to identify application requirements.

Table 62-5 describes the formula used to size default swap areas by the Solaris 2.x installation program. These default swap sizes are a good place to start if you are not sure how to size your swap areas.

See Chapter 30, Configuring Additional Swap Space (Tasks) for information about managing swap space.

Buffer Resources

The buffer cache for read and write system calls uses a range of virtual addresses in the kernel address space. A page of data is mapped into the kernel address space and the amount of data requested by the process is then physically copied to the process' address space. The page is then unmapped in the kernel. The physical page will remain in memory until the page is freed up by the page daemon.

This means a few I/O-intensive processes can monopolize or force other processes out of main memory. To prevent monopolization of main memory, balance the running of I/O-intensive processes serially in a script or with the at command. Programmers can use mmap(2) and madvise(3) to ensure that their programs free memory when they are not using it.