How Much Ram Do You Need?

Change isn’t always a good thing, so we’ll begin with some perspective. If you’re not a power user and you only need your Mac for simple applications like word processing and web browsing, a memory upgrade might not be worthwhile. These days, 2GB to 4GB of RAM is plenty for such users, so the amount of RAM you selected for your Mac at purchase could be enough.

For more demanding users -- or anyone who skimped on RAM to dodge Apple’s high memory prices -- a bump in memory can boost performance on your Mac. But don’t go hog-wild; you should only install more than 4GB if you’re running 64-bit applications in the compatible 64-bit Snow Leopard (or Lion!). Most Mac users have their systems default into 32-bit, so upgrading beyond 4GB won’t produce a tremendous performance difference because 32-bit operating systems can only address a max of 4GB of RAM.

Given all that, how much does your Mac really need? To suss out the sweet spot, we began by conducting tests with the benchmarking suite Geekbench, which delivers an overall “score” of memory performance (see chart, left). Unfortunately, those scores don’t clarify differences in performance as much as we’d like (especially between the 4GB and 8GB 32-bit MacBook Pro), mostly due to the limitations of the memory controller. That’s because when all slots are filled with RAM, there’s a slight degradation in memory bandwidth due to the way the memory controller has to access all of the DIMMs.

The difference between mobile memory sticks (SO-DIMMs; bottom) and (DIMMs; top) desktop memory sticks is more than just their size.

However, we found a significant boost when 64-bit software comes into play. Our final and most conclusive test involved timing how quickly Photoshop CS5 could perform a beefy batch-process on a 13MB file. When we bumped up the RAM from 4GB to 16GB, we found a major difference of more than 19 minutes! The take-away: there’s a definite benefit to plopping in more sticks of RAM if you need to work with RAM-intensive applications since the performance of those apps is highly dependent on how much memory you have available.

That also means that deciding how much RAM to put in your Mac is more of a personal decision based on what you use your machine for, rather than a problem that can be solved by hard data and numbers. To help you make that call, we exchanged emails with experts in the RAM industry and gathered their advice. Mark Tekunoff, senior technology manager at Kingston Technology, suggests that if you’re running a 64-bit operating system and applications, at least 8GB of memory is a good option for future-proofing. If you’re not a power user and only need your Mac for simpler tasks, a good 4GB of RAM will get you pretty far.

But if that means an upgrade is in order, do you need to buy Mac-specific RAM? No. Robert Pearce at Corsair explains: “There’s no physical difference between the DRAM you use in a PC and the DRAM you use in a Mac. You just need to make sure you’re purchasing RAM that’s compatible with your system.”

There’s no physical difference between PC and Mac RAM.

Likewise, there generally isn’t a significant difference between name-brand and generic RAM because many companies buy their RAM from the same factories and stamp it with their logo. Still, we think it’s better to stick with name-brand companies that are vested in ensuring your memory works as intended, especially since they offer the added convenience of a customer-service hotline in case you’re sold faulty RAM.

Diagnosing Bad RAM

Before you start digging up screwdrivers to open up your Mac, you should determine if you genuinely need a RAM upgrade. After all, even if your Mac has slowed to a crawl lately, it might not be because the memory is topping out. If you’re having performance issues with your Mac -- say, applications seem to crash randomly when you start them up, your Mac is incessantly freezing, or some software just hangs without really loading -- you’ll want to check to see that the RAM is really the issue and you’re not simply in need of a software update.

To do that, run Memtest, an advanced memory diagnostic tool that runs a Unix command to analyze your RAM. The blog Command-Tab put together a Memtest unloader (bit.ly/memunloader) that places the utility right into your /user/bin folder. After you install it, you’ll need to reboot your Mac into Single User Mode by holding down Command-S during the boot screen. After system logging is done scrolling, type “memtest all 2” to test your memory twice. Memtest will run two passes to see if there’s anything wrong with the RAM (it’ll take just a couple minutes).

Memtest runs a Unix command to check your RAM.

Alternately, you could run the test for a couple of hours with the command “memtest all” if you suspect that the issue is an intermittent memory problem. (By not setting a number after “all,” you’re telling Memtest to loop indefinitely.) If your system locks up or freezes during that test, there’s probably some bad memory in your Mac, and it’s time to replace it. When you’re all finished, type in “exit” to boot normally into OS X.