Apple's new 8-core Mac Pro may be throttled by its memory technology and its older operating system, according to new benchmarks run on the cutting-edge system. Even though the eight total Xeon cores offer a literal doubling of theoretical CPU performance in testing, testing of memory-intensive programs such as Aperture and Photoshop CS3 -- both of which routinely store gigabytes of images in memory -- reveals almost negligible differences between 8-core and 4-core 3GHz systems.The seeming deadlock may be attributable to a lack of real-world bandwidth, photographer Lloyd Chambers writes.
"Memory bandwidth is inadequate for 8 cores. It’s already a limiting factor with the current quad-core 3.0 GHz Mac Pro," he says. "Memory copy speed is at best 2.9GB/sec on the Mac Pro, in spite of Apple’s highly misleading claims of 21.3 GB/sec... That’s a measly 700MB/sec per core on a quad-core machine, and only 350MB/sec per core on an octa-core machine. By comparison, a 6-drive hard disk RAID array can easily perform at over 400MB/sec!"
It was unclear whether the bottleneck was completely endemic to Intel's current Xeon 5100 (dual-core) and 5300 (quad-core) platforms or a flaw in Mac OS X memory handling.
More directly to blame, however, is Mac OS X Tiger's mishandling of CPU resources. The operating system engages in a bad practice known as "core swapping," Chambers says. The Apple software, released in 2005, is frequently unsure of when to keep a particular task on a particular core and sends the relevant data to another one of the eight cores, saturating the already constrained memory pipeline with redundant information.
Testers of the new Apple workstation are suggesting that buyers carefully evaluate the programs they run before buying the $1500 upgrade in the current software enviroment, as the extra resources were said to translate to "multi-threading muscle" only in certain circumstances.
Mac OS X Leopard may potentially solve the problem, suggests Chambers. The computer builder hasn't officially announced explicit support for more than four cores in its operating system.
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