Apple Mac Pro - How Much Memory is Enough?

In the first edition of this month’s Affordable HD enewsletter, we keep the focus on the Apple Mac Pro, specifically analyzing the optimal memory configurations for editing and encoding with Apple Final Cut Studio, Adobe Creative Suite 4, and Telestream Episode Engine. I looked at several scenarios.

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Figure 1. The Apple Mac Pro’s memory tray.

First, I tested performance at 8GB, 12GB, and 24GB of memory to understand where the sweet spots might be for all involved applications. Then, to assist those trying to identify the optimal memory configuration for Mac Pros, I tested performance at 8GB and 12GB capacity, using different combinations of memory to achieve each total. Specifically, at 8GB, I compared the performance of two 4GB dual in-line memory modules (DIMMs) against four 2GB DIMMs, and at 12GB, I compared the performance of three 2GB DIMMs against the combination of two 2GB DIMMs and two 4GB DIMMs.

Why bother with these latter tests? Because the Mac Pro’s new Intel Nehalem chipset has a 3-channel integrated memory controller, which, at least in theory, makes the optimum memory configuration equal amounts of RAM in each of the first three DIMM slots. To be clear, as you can see in Figure 1, the Mac Pro’s memory tray, there are eight memory slots on a dual-CPU system (four on a single-CPU system). In the figure, there’s a DIMM in the first three slots on each side, while the fourth is empty.

Several websites have reported that performance can actually drop when you configure RAM outside this optimal configuration. For example, Bare Feats reported that adding a fourth DIMM to the mix can actually slow performance by up to 33 percent. Why? In theory, because adding the fourth DIMM forces the CPU to split the memory bandwidth between the third and fourth DIMMs, which drops the effective throughput to those DIMMs by 50 percent.

That said, Bare Feats is clear that the test used to produce these results, the digLloydTools (DLT) stress test, is an artificial, worst-case test. It concludes: "DON'T PANIC: If you have memory installed in all eight slots of your eight-core Nehalem (or all four slots of a four-core Nehalem), it may not penalize your real-world application performance. The vast majority of real-world applications do not saturate the memory bandwidth."

I didn’t want to duplicate these tests, but I was interested in how mixed-memory configurations would perform given the 3-channel memory controller and the fact that upgrading all three slots to identical RAM configurations may not be the most affordable approach. But I get ahead of myself. Let’s begin at the beginning.


Comments (2)

Avery
Said this on 9-21-2010 At 12:36 pm

Thanks for your efforts and reporting your findings -- good stuff. I have a 2009 MP, with 4x2GB and am trying to decide between going to 6x2GB, 8x2GB, or 6x4GB config. The last, being 'optimal', but probably overkill for me and 8x as much as option 1. I am running CS5, where Premiere Pro is now 64-bit, versus 32-bit in your test. I'm curious if this would have any impact on the results of your study.

Said this on 9-21-2010 At 08:40 pm

Hey Avery:

Thanks for note - in general, the Mac is very efficient with low memory configs. If you're working with DV and HDV, I wouldn't expect much difference between 8 and 24. If H.264 from a DSL, though, you'll see much more.

Send your email address to jozer@mindspring.com and I'll send you a couple of slides from a presentation I did on this.

Jan

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