Paging is the process of exchanging pages between the physical memory and the secondary storage in the following cases:
- image activation
- the process's demand for pages is greated than the number of pages available in the working set
- when the page cache fills up (swapping)
One or more page files are used for paging.
A page fault occurs when a required page is not in your balance set (primary cache).
- A hard fault requires a read operation from a page or image file on disk.
- A soft fault involves mapping to a secondary page cache; this can be a global page or a page in the secondary page cache.
Excessive paging is considered to be a performance problem.
Page fault rate varies as a function of the working set size for a program. For initial working set sizes that are small relative to the demands of the program, a small increase in working set size results in a very large decrease in page fault rate. Conversely, as initial working set size increases, the relative benefit of increased working set size diminishes. A point may be reached at which further substantial increases in working set size yields little or no appreciable benefit. 
Paging problems typically occur when the secondary page cache (free-page list and modified-page list) is too small. This systemwide cache, which is sized by AUTOGEN, should be large enough to ensure that the overall fault rate is not excessive and that most faults are soft faults. When evaluating paging activity on your system, you should check for processes in the free page wait (FPG), collided page wait (COLPG), and page fault wait (PFW) states and note departures from normal figures. The presence of processes in the FPG state almost always indicates serious memory management problems, because it implies that the free-page list has been depleted. Processes in the PFW and COLPG states are waiting for hard faults (from disk) to be satisfied. Note, however, that while hard fault waiting is undesirable, it is not as serious as swapping. An average free-page list size that is between the values of the FREELIM and FREEGOAL system parameters usually indicates deficient memory and is often accompanied by a high page fault rate. If either condition exists, or if the hard fault rate exceeds the recommended percentage, you must consider enlarging the free- and modified-page lists, if possible. Enlarging the secondary page cache could reduce hard faulting, provided such faulting is not the result of image