Essential System Administration, 3rd Edition

Managing Memory

Memory resources have at least as much effect on overall system performance as the distribution of CPU resources. To perform well, a system needs to have adequate memory not just for the largest jobs it will run, but also for the overall mix of jobs typical of its everyday use. For example, the amount of memory that is sufficient for the one or two big jobs that run overnight might provide only a mediocre response time under the heavy daytime interactive use. On the other hand, an amount of memory that supports a system’s normal interactive use might result in quite poor performance when larger jobs are run. Thus, both sets of needs should be taken into consideration when planning for and evaluating system memory requirements.

Paging and swapping are the means by which Unix distributes available memory among current processes when their total memory needs exceed the amount of physical memory. Technically, swapping refers to writing an entire process to disk, thereby freeing all of the physical memory it had occupied. A swapped-out process must then be reread into memory when execution resumes. Paging involves moving sections of a process’s memory—in units called pages—to disk, to free up physical memory needed by some process. A page fault occurs when a process needs a page of memory that is not resident and must be (re)read in from disk. On virtual memory systems, true swapping occurs rarely if at all^[19] and usually indicates a serious memory shortage, so the ...