Apr 4, 2005

Photoshop CS2-How much RAM? – Fact

Adobe® Photoshop® CS2 will be able to go beyond the previous 2gig/process barrier that pervious versions of Photoshop were limited to. The answer of “How Much Beyond” varies. . .

On OS X with the supported system versions, Photoshop CS2 can go to 3+ gigs. The actual amount could be 3.5 gigs or so, but due to certain reporting issues, it’s tough to say precisely. Tiger (10.4) will not expand beyond this limit since at the root, OS X is still really only a 32 bit OS with certain 64 bit optimizations and Photoshop CS2 is still a 32 bit application for both Mac & Win.

For Windows, the answer is more complicated. If you are running Win 2K or XP Home/Pro with SP1, the limit is still 2 gigs. With XP Pro & SP2, the limit is 3 gigs -IF- you’ve enabled the 3 gig boot switch, which can be problematic.

If you are running the released version of Windows XP 64 bit edition and are running either an AMD64 or Intel EM64T chipset, the limit will be 3+ gigs or so with the same issue of reporting issues.

That is not to say that Photoshop and your OS will not benefit by having more ram. More is always better (unless you have motherboard or ram chip issues). Having an excess of ram beyond what Photoshop can directly use will still benefit the system by reducing system paging and additional applications running will still be able to use the ram. With Photoshop CS2, the engineers are doing some special OS caching.

From Russell Williams, Photoshop architect:

Just to be a little more explicit on the “3+GB thing” — if you’ve got 4GB and are still hitting the scratch disk on either Mac or Win, you will probably see significant benefit from adding RAM. We’ve seen 40% and greater speedups when running tests on big documents that hit the scratch disk by increasing RAM from 4GB to 6GB.

What’s happening is that Photoshop can only make direct use of about 3.5 GB of RAM on Mac and 2GB on Windows and 3+ on XP 64 bit. But when it goes to write to the scratch file, normally that I/O is done directly from Photoshop’s memory to the disk (or vice versa). If you’ve got more than 4GB of RAM, we let the OS do its buffering thang instead. That is, data is not written directly to the disk; the OS copies it into RAM buffers that reside in that extra RAM Photoshop can’t otherwise use. If Photoshop later asks for some data from the scratch disk and it happens to be in one of those buffers, the OS copies it from the buffer instead of reading from the disk.

This is the usual way that most file access works — via the OS disk buffers. The reasons Photoshop normally *doesn’t* use those buffers are:

1. It costs extra to copy the data to and from the buffers instead of just reading it to / from the disk. If you get data from the buffer instead of doing a disk I / O, this is more than worth it, but…

2. Photoshop’s access patterns to its scratch file mostly don’t match what the OS is expecting. The OS assumes that if you just read or wrote it, you’re likely to need it again soon. But that’s not generally the case with Photoshop’s scratch disk.

But when you’ve got more RAM than Photoshop can use directly, there’s little to lose by letting the OS cache it.

So yes, 4 gigs or more can be of benefit to Photoshop CS2.

The default ram allocation settings are 70% for Mac and 55% for Windows. Under normal use for most users, these settings will be optimal. You can fine tune your own settings based upon your own system, installed ram and the way in which you use Photoshop. Depending upon the number of system processes running, and the number of other applications running, you can try turning up the ram allocation percent incrementally upwards while checking the available unused ram ram with a system utility. Ideally, you should always leave a few hundred MBs free to avoid starving the system. For Mac, you can use Activity Monitor (built in OS X) to watch ram usage. For Windows you can watch Performance Monitor which is built in.

Ram is only one of the three major Photoshop bottlenecks; CPU speed, ram and scratch disk. Multiple processor machines are faster than single processor machines and Photoshop, since version 4 or so can use multiple processors on many Photoshop operations.

The old saying, “you can never be too thin, or too rich or have too much ram” is basically true. Adding ram will, in most cases help. You can set your window display option to display “Efficiency” and watch it during the course of normal Photoshop operations. If your Efficiency remains at or near 100%, you basically have enough ram. If however, you notice Efficiency dropping below 90% for any sustained period, you are a candidate for adding ram. The ram requirements vary considerably based upon file size, number of files open at a time and the particular Photoshop operation running. Some operations are particularly ram intensive. In some situations, the available ram can become so fragmented that system and Photoshop performance can degrade. In that situation, closing down unused applications and quitting and re-launching Photoshop can restore the fragmented ram.

It should also be noted that the Cache Level settings under the Memory and Image Cache can have an impact on ram usage. Increasing the Image Cache setting will speed screen redraw-particularly when you are working with larger files with a lot of layers. However, the Image Cache doesn’t do too much for small files. The Photoshop CS2 default is set to Cache Level 6 with 8 being the maximum. If you routinely work with larger multi-layered files, try increasing the cache level.

The 3rd Photoshop bottleneck is scratch disk usage. For optimum performance, the primary scratch disk for Photoshop should be a different physical drive than your system’s boot drive (assuming that your system is set to page to your boot drive). Having a second fast drive for Photoshop’s scratch disk can improve performance. There is simply no longer a fast and hard rule about how large the scratch disk should be. It depends upon too many factors such as file size, ram allocation and what your History States setting is. It should be noted that History States do -NOT- affect ram allocation, it only affects the potential for scratch disk size. The higher the History States, the larger the potential scratch disk size you’ll require-sometimes by a huge amount. The scratch disk can be split across up to 4 different drives, however, I’ve found it’s optimal to assign a single large volume rather than several smaller volumes. While the previous largest file size on most systems was 2 gigs, this barrier has been eliminated in Photoshop CS and above because Photoshop can split the scratch file size into hundreds of individual files.

There is an upwards limit to the scratch file “somewhere around 64 Exabytes, but you’ll run out of ram before you get there so the practical limits is more like 32 Terabytes” according to Photoshop engineer Chris Cox.

Performance for Photoshop CS2 will be considerably better for the Photoshop power user. However, for optimum performance, all aspects of Photoshop bottlenecks must be addressed. Additionally, Photoshop’s performance and stability is still only as good as the health of your system and hardware. Regular maintenance and running a clean system without a load of system hacks and overloading the running applications is the best way to assure maximum Photoshop performance.

For more information about Adobe Photoshop CS2 check the Photoshop CS2 page. Check here for the Photoshop CS2 System Requirements.

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