Source: Adobe Photoshop Support Knowledgebase

How Photoshop allocates RAM

When Photoshop starts, it allocates, or sets aside, a portion of RAM, the size of which is based on the amount that is designated in the Photoshop Memory & Image Cache preferences. While this RAM is reserved for Photoshop processes, it may be used by other applications when it isn’t actively in use by Photoshop, and is released when you quit Photoshop.

If the percentage of RAM allocated to Photoshop is too high, both the operating system and Photoshop can swap pages out of RAM, causing slow performance in Photoshop. Page swapping is a normal operating system function that only affects performance when the amount of RAM that Photoshop and the operating system are trying to use is more than the total amount of RAM on the computer. If you experience slow performance, reduce the amount of RAM assigned to Photoshop to 50% or 60%.

Photoshop can only use the first 2 GB of RAM on the computer. This limit is based on hardware and operating system limitations. The operating system uses some of this RAM, so the Photoshop Memory Usage preference displays only a maximum of 1.6 or 1.7 GB of total available RAM.

If you have 4 GB or more of RAM, set the Maximum Used By Photoshop in the Memory Usage section of the Memory & Image Cache preferences to 70%. This amount ensures that Photoshop uses as much of the first 2 GB of RAM as possible, while reserving enough RAM for other applications.

Scratch disk sizes and RAM

Photoshop CS may allocate more memory (RAM and scratch disk) to itself when starting than previous versions, depending on the amount of RAM on the computer and the percentage of RAM allocated to Photoshop in the Memory Usage preference. The amount of available RAM on the system also dictates the image tile size in Photoshop (Photoshop uses tiles to redraw images in sections).

Photoshop displays RAM and scratch disk usage and allocation in the Scratch Sizes section of the status bar. The number on the left indicates the RAM used and scratch disk space allocated to Photoshop. The number on the right indicates the amount of RAM that Photoshop can use for image storage or scratch disk. Photoshop CS uses more memory when it starts than previous versions. Thus, the number on the left may be larger than what you are accustomed to from previous versions, but does not indicate that images use three times more RAM than they did in previous versions.

Read the support document.

This story was posted on Monday, November 29th, 2004 at 12:29 am and is filed under Photoshop "Issues". You can follow any responses to this story through the RSS 2.0 feed. Both comments and pings are currently closed.