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.

About Jan Ozer

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I help companies train new technical hires in streaming media-related positions; I also help companies optimize their codec selections and encoding stacks, and evaluate new encoders and codecs.

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