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Power Mac G5s: Two brains, one chip

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“Choose a Power Mac G5 Dual or Power Mac G5 Quad and prepare to be blown away,” Apple’s online store boasted when the new pro desktops went on sale.

The copy writers may have a point about the Quad, but when it comes to the 2GHz and 2.3GHz dual-core models, a couple of grains of salt are in order: In terms of overall performance, these new machines look, feel, and perform pretty much like the models they replace. But they have more memory capacity than ever; a new, superfast bus for expansion cards; and graphics capabilities that are sure to appeal to gamers and animators, as well as many scientists and engineers.

1x2 = 2x1

The new machines feature CPUs with two G5 processing engines on a single sliver of silicon—each dual-core processor has two of everything that defines the G5, including its Velocity Engine. That’s something completely new for the Mac, though IBM, Advanced Micro Devices, and others have been using a similar approach on other chips for several years.

The idea of combining two processing engines to boost performance, however, is not at all new to the Mac. The first dual-processor Power Mac appeared way back in 1997, and Apple has consistently offered such systems for the last five years. (Remember the slogan, “Two brains are better than one”?)

The difference is that this benefit used to require two separate, complete CPUs; now, with dual-core processors, Apple can include two brains on just one chip. Put simply, G5s with one dual-core processor replace G5s with two single-core processors; in the new dual-core models, even the clock speeds are the same. OS X apparently doesn’t see much difference: Apple’s System Profiler reports two CPUs, just as in dual-processor systems, and the Activity Monitor works the same way with two cores as it does with two processors.

(The Power Mac Quad, by contrast, has two processors with a dual core each, or four processing engines in all—twice as many as any previous Mac. That’s the system that really might blow us all away, but it wasn’t available for review at press time.)

Adding it all up

Switching from dual processors to a dual-core single processor presumably saves Apple a little money, though the company isn’t yet passing any savings on to customers: at $1,999 and $2,499, respectively, the 2GHz and 2.3GHz Dual models are priced the same as the two-processor Power Macs they replace.

The switch also saves space and simplifies the challenge of cooling the machine, but here, too, the benefits aren’t yet apparent: the dual-core models have the same big enclosures as their predecessors, with the same elaborate thermal-management system. In fact, there are still two large fans mounted in the front of the processor module, even though there’s now only one chip inside.

Of course, there are some differences between the new and old designs. Each core now has 1MB of Level 2 (L2) cache for itself; in the previous Power Mac generation, each CPU had only 512K of L2 cache. On the other side of the ledger, each CPU in the dual-processor models had its own high-speed frontside bus for fetching and returning data and instructions; now the two cores on a single chip have to share the same bus.

To add one more variable to the equation, the new Power Macs also take advantage of the latest in DRAM technology. They use 533MHz DDR2 (double data rate 2), also known as PC2-4200, memory, compared with the 400MHz DDR chips of the previous models.

The net effect of all these changes on actual performance? Positive but nothing to write home about, according to our benchmark tests. In our Speedmark 4 suite and in most of our other standard tests, the 2.3GHz dual-core Power Mac G5 outpaced the 2.3GHz dual-processor, but only slightly (see the benchmark chart). Comparing the new 2GHz dual-core model to the previous 2GHz dual-processor model produced similar results.

New slots and new graphics cards

The one test in which the 2.3GHz dual-core system left its 2.3GHz dual-processor predecessor in the dust—and even outperformed the 2.7GHz dual-processor model—was our Unreal Tournament 2004 frames-per-second test (see the benchmark chart). These stellar results don’t seem to have anything to do with the new G5 processors—they’re really a function of the new graphics cards that come with the dual-core systems and the new high-speed slots they sit in.

In the case of the new 2.3GHz Dual (as well as the Quad), the standard card is the Nvidia GeForce 6600 with 256MB of video RAM. The stripped-down Nvidia GeForce 6600 LE, with 128MB of memory, is standard in the new 2GHz Dual, but if you order from Apple’s online store, you can upgrade to the full version for just $50. Both cards have two DVI connectors, one of which is the dual-link variety that’s required for Apple’s 30-inch Cinema HD Display.

If that’s not good enough, Apple is offering two other Nvidia upgrade options: the GeForce 7800 GT ($400 for the dual-core 2GHz and $350 for the dual-core 2.3GHz), which promises even faster motion graphics and animation; and the Quadro FX 4500 ($1,700 for the 2GHz and $1,650 for the 2.3GHz), which has 512MB of memory and two dual-link ports, plus an extra connector for stereo 3-D goggles.

All these cards take advantage of PCI Express (PCIe), a new (to the Mac) industry-standard architecture for add-on cards. The new slots offer much better throughput and greater flexibility than the AGP, PCI, and PCI-X slots they replace.

The problem is what to put in them. Besides the Nvidia video cards, Apple sells a PCI Express (PCIe) Fibre Channel card, and Blackmagic Design and AJA Video Systems have released high-bandwidth PCIe video-capture cards for the new Macs. Digidesign says it plans to release PCIe-compatible Pro Tools|HD systems by the end of November. Other cards, particularly for video and audio, but also for 10GB Ethernet and other demanding applications, will undoubtedly appear soon. But for now, there aren’t nearly as many choices as Mac professionals have been used to with previous-generation Power Macs.

And don’t even think about transferring your existing PCI and PCI-X cards. Although its name makes PCI Express sound like an evolutionary heir to the venerable PCI standard, it’s actually a fundamentally different technology, and there’s no compatibility between old cards and the new slots. Apple is following its usual abrupt approach to technology transitions: out with the old, in with the new. (For people who require compatibility with PCI-X, Apple is keeping the 2.7GHz dual-processor Power Mac G5 on the market, at least for now, at the reduced price of $2,799.)

With regard to software, the PCIe standard was designed to cause no compatibility issues, but a few applications that specifically look for an AGP graphics cards won’t run on PCIe systems. One notable example is Apple’s own Final Cut Pro, versions 4.0 through 4.5. Apple’s gives you only one solution—upgrade to version 5.0.3, for $399!

Wait, there’s more

Other notable changes in the new Power Macs include:

• 512MB of memory is standard, but you can now boost total memory all the way to 16GB, double the previous maximum. (That goes for both Dual models—for a change, Apple hasn’t imposed any artificial limitations on the lowest-priced version.) And the machines can take advantage of ECC (error-correcting code) memory, an expensive form of RAM that can automatically correct certain memory errors—a technology some government, corporate, and academic customers insist on for mission-critical and computer-intensive applications.

• The internal 56K modem, made optional in the last Power Mac generation, has now disappeared altogether; if you need dial-up capabilities, your only choice is an external USB modem (Apple’s costs $49).

• In exchange, both dual-core models have two Gigabit Ethernet connectors, so you can dedicate one to a storage-area network (SAN) or an isolated management network in addition to a conventional network. Both ports support an advanced throughput-boosting networking technology known as Jumbo Frames (packets of up to 9,000 bytes).

• Both Dual systems come with Apple’s four-button Mighty Mouse.

Dual-Core Power Macs Tested

Speedmark 4 Adobe Photoshop CS2 Cinema 4D XL 9.1 Compressor 2.0 iMovie HD iTunes 6.0.1 Unreal Tournament 2004
Power Mac G5/2GHz dual-core 208 1:04 1:23 6:20 0:36 0:58 40.6
Power Mac G5/2.3GHz dual-core 226 0:56 1:11 5:35 0:33 0:52 50.1
Power Mac G5/2.3GHz dual-processor 224 0:59 1:13 6:03 0:36 0:52 40.2
Power Mac G5/2.7GHz dual-processor 248 0:52 1:03 5:12 0:25 0:46 47.7
>Better <Better <Better <Better <Better <Better >Better

Best results in red. Reference system in italics .

Speedmark 4 scores are relative to those of a 1.25GHz Mac mini, which is assigned a score of 100. Adobe Photoshop, Cinema 4D XL, iMovie, and iTunes scores are in minutes:seconds. All systems were running Mac OS X 10.4.2 with 512MB of RAM, with processor performance set to Highest in the Energy Saver preference pane. We converted 45 minutes of AAC audio files to MP3 using iTunes&#8217; High Quality setting. We used Unreal Tournament 2004’s Antalus Botmatch average-frames-per-second score; we tested at a resolution of 1,024 by 768 pixels at the Maximum setting. The Photoshop Suite test is a set of 14 scripted tasks using a 50MB file. Photoshop’s memory was set to 70 percent and History was set to Minimum. To compare Speedmark 4 scores for various Mac systems, visit our Apple Hardware Guide .—Macworld Lab testing by James Galbraith and Jerry Jung

Macworld’s buying advice

Compared with their predecessors, the Power Mac G5 Dual models offer an array of new workstation-class capabilities, including impressive improvements in graphics, advanced networking, enormous memory capacity, and state-of-the-art expansion slots. Between the two models, the 2GHz machine, even with video card and hard drive upgrades, is a slightly better bargain. Of course, you give up a bit of speed, but I suspect that users who demand the ultimate in Mac performance will be saving their pennies for a Power Mac G5 Quad anyway.

[ Henry Norr is a former editor of MacWeek . He has been reviewing Mac systems since 1986. ]

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