Page Swapping

Image files can vary greatly in size. As they increase in size they require more hard drive space for

storage, and they occupy more RAM when opened. The installed RAM available to both Photoshop and the
Mac OSX operating system can be used up quickly when working with large image files. When all available
RAM is already in use but more is required- Photoshop must actively use the scratch disk as a substitute for
RAM.

Mac OSX always reserves a certain amount of RAM for itself- regardless of other running applications.
But when the amount of RAM Photoshop and Mac OSX are trying to use is more than the total amount of RAM
on the computer, Mac OSX also has to use the hard drive as a substitute for RAM, and it begins to actively
read and write data to it’s “virtual memory” swapfile in order to complete whatever operation is in progress.
This is known as “page-swapping”, and has a negative impact on performance. Under ideal conditions, page-
swapping is infrequent or does not occur. However- sometimes its occurrence is unavoidable- especially when
working with really large images. The best way to minimize the possibility of page-swapping is to install a lot of
RAM. The best way to minimize it’s negative effect when it does occur- use a fast hard drive for the Startup
disk.

In the illustration above, active page-swapping is represented by the second number (to the right of the
slash mark) next to Page ins/outs: In this case- 2474. When no page swapping has occurred between restarts
of the computer, this number is normally 0. Here, all the installed RAM is being used, and more was clearly re-
quired.