Right, I forgot that it was about Photoshop only - Sorry.
No problem. It's a useful reminder that Adobe should get around to fixing this.
From Adobe support website it looks like Photoshop can really take advantage of more than 4GB of RAM:
No, it's not Photoshop taking advantage of it, but the operating system.
..If you have more than 4 GB (to 6 GB (Windows) or 8 GB (Mac OS)), the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop, is now cached in this high RAM before being written to the hard disk by the operating system...
I've never bothered to perform tests on my Macs to see if the ones with 8GB are really that much faster than the ones with only 4GB.
It depends on your usage patterns.
This has to do with the virtual memory subsystem of MacOS X. While I don't know the specifics of how this program works, I'm pretty certain that it doesn't prioritize file system caching over applications.
So if you e.g. run a web browser for a couple of weeks, use three or four instances of Bridge, or maybe Capture One (or goodness forbid, Aperture), these applications will be competing for the remainder of the RAM.
I won't call Adobe's advice bad
, but it's certainly not the most helpful, since it seems to imply that Photoshop is getting preferential treatment to that extra available RAM. As far as I know, this is simply not the case.
If you want to guarantee that RAM is dedicated to improving Photoshop scratch disk performance, create a ramdisk.
Here's how to create a 4 GB ramdisk on the command line (you probably need to do it differently to ensure that it happens at every boot, though); lines beginning with the $ sign is the command, and the following line(s) is(are) the output from the command:
$ hdid -nomount ram://8388608
$ hdiutil mount /dev/disk1
The number after ram://
is the number of 512 byte sectors, so if you multiply it by 512, you get the number of bytes in total for the ramdisk. A 2 GB ramdisk would be ram://4194304
(Thanks to Phil Armstrong for providing this particular answer; I'm not really familiar with MacOS X yet, though I know how to go about it with Linux.)
You can then use /Volumes/untitled for your scratch disk space.