Power Mac G5 Quad: Fast performance at its core

After Steve Jobs announced Apple’s plan to drop the PowerPC processor and put Intel inside future Macs, some performance-hungry pros 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 (Best Current Price: $3189.00), 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 and similarly demanding environments.

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 2.0GHz dual-core Power Mac G5 (Best Current Price: $1898.00) and the 2.3GHz dual-core Power Mac G5 (Best Current Price: $2284.00), 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 onetime clone-maker DayStar Digital marketed a system 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 tradeoff 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.