Power Mac G5 Quad

After Steve Jobs announced Apple’s plan to drop the PowerPC processor and put Intel inside future Macs, some performance-hungry professionals feared they’d see no major advances in Mac processing speed until the arrival of an all-new, Intel-based line of high-end desktop models – something that could be a year or more away, according to the company’s transition timetable.

Cross that one off your worry list. With the introduction of the Power Mac G5 Quad 2.5GHz, Apple has delivered the biggest jump in Mac performance in years – not for every application, but the type for which this machine was designed. The Quad excels at the computation-intensive, multiprocessor-savvy programs commonly used in fields such as scientific computing, professional audio and video editing and similarly demanding environments.

Gang of four
The speed breakthrough is not the result of a sudden surge in G5 clock speed – two and a half years after the first G5s debuted at speeds of up to 2GHz, IBM, Apple’s chip supplier, still hasn’t hit the 3GHz mark. In fact, at 2.5GHz, the Quad’s two G5s actually run a shade slower than the 2.7GHz chips that powered the previous top-of-the-line Mac.

The difference is that the Quad’s processors are a new version of the PowerPC that puts two G5 processing units – known as ‘cores’ – on each chip. Each core has all the features of previous G5 chips, including a Velocity Engine to accelerate scientific and multimedia data processing, a 32K Level 1 cache for ultra-fast access to recently used data, and an equally speedy 64K Level 1 cache for instructions. In addition, each core in the new G5 chips has a full megabyte of Level 2 cache memory, compared to 512K per chip in the previous
G5 generation.

The other two new desktop models announced alongside the Quad, the 2GHz dual-core Power Mac G5 (£1,399) and the 2.3GHz dual-core Power Mac G5 (£1,749), use similar dual-core CPUs, but those systems have only one chip apiece. With two dual-core processors, the Quad has – as its name is intended to suggest – the equivalent of four standard G5 chips, twice as many as any previous Apple system. (Dedicated collectors of Mac minutiae may recall, however, that one-time clone-maker DayStar Digital marketed a system (the DayStar Genesis MP 932+ with four PowerPC 604e chips, at speeds ranging up to 233MHz, for a few months in 1997.)

Just as Mac OS X divides chores between the two CPUs in older dual-processor Power Macs and between the two processing engines in the new single-chip, dual-core G5 systems, all four engines in the Quad can share the load. One technical trade-off in the system’s design. In previous Power Mac G5s, each CPU had its own frontside bus to transport data and instructions to and from memory, while in the new models, the two cores in each chip have to share a bus. But the new chips’ expanded L2 caches help to compensate: with more information already on hand in the cache, the cores don’t need to turn to the bus as often. And when they do, a faster memory system should ensure that the bus gets loaded in a hurry. The Quad, like its single-processor dual-core siblings, uses 533MHz DDR2 (double data rate two), also known as PC2-4200, memory, compared with the 400MHz DDR chips in the previous models.

Net effect
If you give just a quick glance at the Macworld Lab benchmark results for the Quad, you might wonder what the fuss is all about. On the Speedmark test suite, the new system barely managed to edge out the previous Mac performance champ, the 2.7GHz dual-processor model, and considering that the Quad has twice as much raw processing power as its single-chip, dual-core siblings, its lead over them is surprisingly modest.

A closer look at the results, however, shows why the Quad really is a big deal – for the markets it was designed to serve. On many of the real-world tasks that make up the Speedmark suite, the Quad only marginally outperformed the other models we tested; on six of the 15 tests, it even lagged slightly behind the 2.7GHz dual-processor system.

That’s because many of the tasks timed in Speedmark, such as starting up the system, depend more on other components – particularly the hard drive – than on the available processing engines; other Speedmark tests use applications, such as Apple’s own consumer-oriented iTunes and iMovie, that haven’t been optimised for multiprocessor systems. Even our Photoshop CS2 test, a set of 14 scripted tasks using a 50MB image, showed a speed up of less than 10 per cent relative to the 2.7GHz dual-processor Power Mac, even though past testing has shown that Photoshop benefits greatly from dual-processor hardware.

A few of Macworld’s lab tests, however, show the stunning improvements we’d hoped for from the Quad. In our standard rendering test with Maxon’s Cinema 4DXL 9.1, the Quad needed only 37 seconds to handle a chore that took 63 seconds on the 2.7GHz dual-processor Mac, 71 seconds on the recently released 2.3GHz dual-core Power Mac, and 83.3 seconds on the new 2GHz dual-core model. In a separate test (not part of the Speedmark suite) that involves encoding video into MPEG2 format with Apple’s Compressor utility, the Quad got the job done in just 3 minutes and 23 seconds, whereas the 2.7GHz dual-processor system needed 5 minutes and 12 seconds, the 2.3GHz dual-core required 5 minutes and 35 seconds, and the 2GHz dual-core model, at 6 minutes and 20 seconds, took almost twice as long as the Quad.

Those are huge time savings, the kind that can convince even tightwad corporate bean counters to authorise trading up to new hardware. And these aren’t just isolated examples – Apple has a whole series of in-house tests showing the Quad outperforming the 2.7GHz dual-processor model.

The moral of all this: the benefits of the Quad, even more than previous Macs, depend on the applications you run and the tasks you perform most frequently. On some tasks in some programs, those benefits might not justify investment in a Quad. But for applications that have been fully optimised for multiprocessing – especially programs that make heavy use of the G5 Velocity Engine – and for users who try to run multiple computation-intensive programs simultaneously, the Quad may quickly pay for itself in spades.

Processing isn’t everything
Besides, the Quad’s four G5 cores are not its only attraction. As our Unreal Tournament 2004 frame-rate test suggests, the graphics card that’s standard in the machine’s 16-lane PCI Express slot – the nVidia GeForce 6600 with 256MB of dedicated memory – delivers great action video. And the two other nVidia graphics cards Apple offers as upgrade options for the Quad – the GeForce 7800 GT with 256MB of memory (£239) and the two-slot Quadro FX 4500 with 512MB of memory (£1,110), which supports two 30-inch Apple Cinema HD displays plus stereo 3D goggles – undoubtedly perform even better, both with games and with scientific visualisation applications.

In addition, the Quad, like the dual-core Power Mac G5s, offers all the advanced features that users are used to, including FireWire 800 as well as 400 ports; a DVD-R SuperDrive, now running at up to 16x and supporting double-layer burning; and digital optical audio in and out jacks. The Quad also sports major enhancements over previous models that are aimed at users who work on particularly demanding tasks and need the most advanced networking, storage, and expansion technologies available. It has room for up to 16GB of memory (with support for ECC – error-correcting code – RAM if it’s needed), three free PCI Express slots (one eight-lane, two four-lane), and dual Gigabit Ethernet ports, offering support for Apple’s Xsan storage system and for throughput-boosting features such as jumbo frames.

Sound of speed
One note of caution: I found the Quad reasonably quiet in routine operation, but when you push it – in particular, by running tasks that keep all four G5 cores cranking – the fans kick in to assist the liquid-cooling system. You’ll definitely hear, and maybe even feel, the resulting whoosh. The Quad operates within the same decibel range as previous Power Mac G5s, according to Apple, but when you get close to the high end of that range, the noise can be pretty intense.


The value of the Power Mac G5 Quad 2.5GHz depends on what you do with it. With many applications, it’s merely the fastest Mac yet, but not by much, and the difference might not be enough to justify the expense. On the other hand, for users of many high-end media-processing and scientific applications – especially those who also want to use the latest expansion, networking, and storage technologies – the Quad adds up to a tremendous advance in Mac power, one well worth the price.

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