Actual memory available to Photoshop is always less than that, because the system scarfs up some of it.

The graphs below show how much advantage each Mac model has with Photoshop CS5, in terms of maximum memory that Photoshop can actually use. Running other programs will reduce that figure.

Figures/graphs below are approximate and slightly optimistic, each system varies, and not all free memory can really be used in all cases.

Mac Pro

You can install up to 64GB in an Apple Mac Pro. Here’s how.

For large files, the graph shows that the potential performance advantages are huge: CS5 can use 20X the memory of CS4.

Mac Pro up to ~60GB for Photoshop CS5

MacBook Pro

You can install up to 8GB in an Apple MacBook Pro. Here’s how.

On the MacBook Pro, Photoshop CS5 can use only about 1.8 times the memory of CS4, which is plenty for many jobs, but still inadequate for users working with 500MB or larger files.

A MacBook Pro can have 8GB of memory, but I found that virtual memory paging begins right around the 5.5GB mark (allowing CS5 to use that amount). So the real world limit without paging is probably closer to 5GB. Virtual memory thus remains highly relevant on the MacBook Pro, which is why you want an MPG Pro Laptop with dual solid state drives: it offers the fastest possible virtual memory performance.

The solution for big jobs? Get a fast scratch volume and fast virtual memory using a fast solid state drive.

Mac Pro up to ~6.4GB for Photoshop CS5

iMac

You can install up to 16GB in an Apple iMac (current model with 4 memory slots), or 8GB in older recent models. Here’s how.

On the iMac, Photoshop CS5 can use about 4.7 times the memory of CS4, which is plenty for any reasonable size job. The Mac Pro is a far better investment than a dead-end iMac, but this is a major advantage of an iMac over a MacBook Pro.

Mac Pro up to ~14.2GB for Photoshop CS5

The Cache Tile Size setting is a sharp stick in the eye. Use it wrong, and you’ll be a hurtin’.

It’s startling that the recommended default setting yields hugely impaired performance. See the graph below.

Adobe’s online help says: “To quickly optimize these settings, click a preset…”. Nonsense. Might as well jab a sharp stick in your eye.

This is a careless design decision (or maybe an outright mistake) that will hurt many customers by wasting their time needlessly. Something else should be done, up to and including CS5 figuring it out automatically for any particular file (self analysis). But at the least, not offering a sharp stick.

The settings shown at right performed about as well as anything I tried, though there are so many combinations that it’s possible there’s a better set. For general use, I’d probably use a history of 100 or so, because I like history.

The graph below says it all— don’t even think about using 128K tiles with big files.

Time in seconds, lower is better

Like the program itself, plugins must be 32-bit or 64-bit, to work with a 32-bit or 64-bit Photoshop CS5.

Many 3rd-party plugins are not yet 64-bit, which means they won’t run at all in CS5 in 64-bit mode.

The workaround is to use CS5 in 32-bit mode (see below), but there is a downside: unlike Photoshop CS4 which could use up to 3072MB memory, CS5 limits itself to 2100MB. This limitation is fine for moderate size files, but will be a performance hit for large files.

If you’re a 3rd-party plugin user, stick with Photoshop CS4 until the plugin issue resolves (by vendors releasing 64-bit version).

You can run both Photoshop CS4 and CS5.

CS5 can be forced to run in 32-bit mode, but won’t use more than 2100MB memory

The Adobe download page for the CS5 plugins is at http://kb2.adobe.com/cps/828/cpsid_82824.html.

The Mac download is here.

http://macperformanceguide.com