Will PS CS4 handle GigaPixel files?

September 25, 2008

With the current announcement of CS4 there’s a healthy discussion of the latest improvements Photoshop’s processing abilities as well as its new capabilities, over on the Prodig email List.

Well we thought you might like to read this insight that Russell Williams, one of the Adobe team, gave in response to a recent question…

Q. Will it handle GigaPixel files?And will it make good use of 8GB ram/ 8 processor computers with 4 raid discs?

A. The quick answer is that Photoshop CS4 is somewhat faster at opening big files in most configurations, but the big win for GigaPixel files will be running the 64 bit version of Photoshop under Vista 64. In that case, Photoshop can take direct advantage of as much RAM as the computer will handle (we’ve tested with up to 32GB), and the performance advantages are huge for huge files. We’ve seen performance improvements of well over 10X in the case of files that required, say, 16GB of RAM.

We’ve made various improvements in handling big files, but the biggest issue is the same as it’s always been: does your entire image fit in RAM. With CS4’s 64 bit version, that’s now possible for images that need more than 3.5GB of RAM. RAID always helps, but if your entire image now fits in RAM, RAID for your scratch volumes becomes less important than it was before just because you’ll be waiting on the scratch disk much less.

The “content aware scaling” feature will take advantage of multiple cores, but I’m not sure what the curve looks like.

“The curve” I’m talking about is the fact that for every Photoshop operation, there’s a curve that shows how much you gain by adding additional processors. As you add more processors, the gain for each additional processor becomes less, and it’s different for each operation. Some operations are 2X faster with 2 processors, 3.5X faster with 4 processors, and 6X faster with 8. Other operations are 1.8X faster with 2 processors, 2.5X faster with 4, and 3X faster with 8. Some operations can’t take advantage of multiple processors at all and so stay the same speed no matter how many processors are in your computer. (And it gets worse — those curves also depend on the memory bandwidth / speed of your computer. Operations that don’t do a lot of arithmetic for every pixel still have to read the original pixels from memory and write the results, so they’re limited by how fast the memory is. Having 8 processors available to do 8 arithmetic operations in parallel doesn’t help at all if they spend all their time waiting on memory access. We expect the next generation of Intel chips to be significantly faster at running Photoshop than their predecessors, not because you’ll be able to cram 16 processors into a box with them, but because they’ll have much greater memory bandwidth).

CS4 has not significantly changed these curves for the vast majority of operations. The very rough guidance I give people is that there are diminishing returns beyond 4 processors. You’ll get some gain by going from 4 to 8, depending on what you spend your time doing. But especially if you process big images, you’re probably better off spending that money on enough RAM to fit your entire image into memory first (especially since more CPUs don’t help at all if you’re spending all your time waiting on disk — 8 processors don’t wait on a disk any faster than 1 processor). And if you spend your time watching progress bars rather than painting and tweaking adjustments as you watch the preview (that is, if throughput when processing big images is more important than responsiveness to small changes), give the Bigger Tiles plugin a try (again, just as with CS3).

What we’ve said before and still say is that the *only* significant advantage to a 64-bit version of Photoshop is the ability to directly address the increased RAM. If your images already fit entirely in RAM with the 32 bit version, the 64 bit version will not run significantly faster.

Just being “64 bit” instead of “32 bit” only increases the speed of the app by about 10% overall — lost in the noise of other changes we make from one version to the next or even changes in the efficiency of the operating system.

It’s all about the RAM and whether you fit.

There will be “performance” and “64 bit” white papers going up on the Adobe web site within a week.

Russell Williams