Adobe Photoshop CS5 for Photographers - Martin Evening [59]
The Memory Usage section therefore provides a guide for the ideal RAM setting for your computer and indicates what the ideal range should be. On a Windows system the default setting is 50% of the total available RAM and on a Mac it is 70% (the figure shown here is now more accurate in Photoshop CS5). Note that if the memory is set too high, Photoshop may end up competing with the operating system for memory and this can slow performance, although as I mentioned earlier, the memory assigned to Photoshop is freed up whenever Photoshop is not running. You can always try allocating 5% above the recommended upper limit, relaunch Photoshop and closely observe the efficiency status bar or Info panel Efficiency readout (Figure 2.19). Anything less than 100% indicates that Photoshop is running short of physical memory and having to make use of the scratch disk space. If you allocate too high a percentage, this can compromise Photoshop's efficiency. So the trick is to find the optimum percentage before you see a drop in performance. You may find the speed test described in the sidebar on page 109 is a useful tool for gauging Photoshop performance.
32-bit and 64-bit RAM limits
The amount of RAM that is available to Photoshop is obviously dependent on the total amount of RAM memory that's installed on your computer. If you run Photoshop on a computer with a 32-bit operating system that has 4 GB or more of RAM installed, a maximum of 2.6 GB of RAM (Mac) can be allocated to Photoshop and a maximum of 3 GB of RAM in the case of Windows systems. Even if you are hit by the 4 GB RAM limit on a 32-bit system and you have more than 4 GB memory installed, the RAM above 4 GB can also be used by the operating system as a cache for the Photoshop scratch data. So although you can't allocate this extra memory directly, installing extra memory can still help boost Photoshop's performance on a 32-bit system.
Assigning scratch disks at startup
If you hold down the keys as you launch Photoshop, this will pop the Disc options on the screen and allow you to add or change the Scratch Disk settings.
Photoshop CS4 saw 64-bit processing enabled for the PC platform and Photoshop CS5 now adds 64-bit support for Macintosh computers too. Providing you have the latest 64-bit enabled hardware and a 64-bit computer operating system, you can install or run Photoshop in 64-bit mode. The main advantage this brings is increased RAM memory access beyond the 4 GB physical limit. In terms of speed you won't necessarily find a 64-bit version of Photoshop to be that much faster than a 32-bit version, except for when you are working with very large files or carrying out memory-intensive operations.
Cache tile size and batch processes
There are certain types of Photoshop operations (such as a Radial Blur filter) that can make heavy demands on the computer hardware, utlilizing nearly all of your processors. If you raise the cache tile size in the Performance preferences to 1024 or 1028K this means the data to be processed is divided into bigger chunks, and this in turn can make a big improvement to batch processing times. The optimum choice of tile size here will depend on whether you are using an Intel or AMD processor (see page 113).
History & Cache
In the History & Cache section the default number of History States is 20. You can set this to any number you like from 1–1000, but remember that the number of histories you choose does have a bearing on the