Eight processing cores working away
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Today marks Intel's first official day of the quad-core processor era with the release of quad-core processors for enthusiasts (the Intel Core 2 Extreme QX6700) and for servers and workstations (the Intel Xeon 5355)--and Intel was kind enough to supply CNET Labs with a pair of 2.66GHz Xeon 5355 processors. As the Xeon 5355 is pin-compatible with the Xeon 5160 processors that came installed in our Mac Pro, we proceeded to swap out the two dual-core processors with the new quad-core processors. (We highly advise you not to try this at home! The Mac Pro case is not designed to allow the end user to perform CPU surgery--and we've got the cuts and bruises to prove it.) With the pair of Xeon 5355 processors installed, we booted the system back up and were greeted with eight active processing cores in both the Mac OS and Windows XP via the Boot Camp Public Beta. With the transplant successful, it was time to run our benchmarks...

Cinebench 9.5
(Longer bars indicate better performance)

Rendering Multiple CPUs

Rendering Single CPU

Mac Pro: 8 cores @ 2.66GHz
(Windows XP)

2106

432

Mac Pro: 8 cores @ 2.66GHz
(Mac OS X)

2070

437

Mac Pro: 4 cores @ 3.0GHz
(Mac OS X)

1604

494

Mac Pro: 4 cores @ 3.0GHz
(Windows XP)

1447

483

Intel QX6700: 4 cores @ 2.66GHz
(Windows XP)

1400

437

Even though dual-core processors have been around for a while now, you'd still be hard-pressed to find many mainstream applications that can efficiently take advantage of both processing cores at the same time (typically referred to as a multithreaded-application). Double that number to four processing cores, and the list of supported multithreaded applications gets even shorter. Double it again to eight...and you get the idea. Some professional multimedia and scientific applications, however, are designed to take advantage of as many processors as are present--and performance will scale accordingly, based on the number of processors available.

Both the Cinebench and PyMOL tests use all available processing cores and hit 100-percent total CPU utilization on every configuration we tested. We saw a 31-percent performance increase on the Mac OS X version of the Cinebench test from the two dual-core chips to the two quad-core chips. Although we doubled the number of cores, we didn't see twice the performance. This is for a few reasons: The quad-core chips are actually running at a slower speed (2.66GHz) than the dual-core chips (3.0GHz). Also, the extra cores introduce some additional computational overhead to the overall workload. Additionally, our "octo-core" rig is our own unsanctioned rig, and therefore isn't benefiting from any of Apple's special sauce, such as firmware and driver updates to better optimize the system for the additional cores.

Our multimedia multitasking test performs a QuickTime encode in the foreground while iTunes is simultaneously encoding in the background. On systems with two or fewer cores, this workload typically saturates the total CPU utilization at 100 percent. With four cores, the system hovered around 40-percent CPU utilization, but dropped to about 23-percent when using eight cores. Interestingly, the actual performance gain we saw between four and eight cores was less than 10 percent. To truly see a significant benefit from the additional cores while performing multiple tasks, you will have to perform a massively multitasking scenario--something we unfortunately did not have time to do for this story.

CPU-limited Quake 4 (in fps)
(Longer bars indicate better performance)

(800x600, low quality, AA off, AF off)

Intel QX6700: 4 cores @ 2.66GHz
(Windows XP)

139.4

Mac Pro: 4 cores @ 3.0GHz
(Windows XP)

124.4

Mac Pro: 4 cores @ 3.0GHz
(Mac OS X)

118.1

Mac Pro: 8 cores @ 2.66GHz
(Windows XP)

114.9

Mac Pro: 8 cores @ 2.66GHz
(Mac OS X)

108.3

Our iTunes and Quake 4 tests are more representative, however, of what you are likely to see with most mainstream applications in a nonmultitasking scenario. The results for both of these tests (as well as with other apps, not shown here, such as Photoshop CS2) indicate that what influences the speed of these tasks is primarily CPU speed. Four cores running at 3.0GHz consistently outperform eight cores running at 2.66GHz. (Note that iTunes is better optimized for the Mac OS, and Quake 4 is better optimized for Windows XP.)

It will be interesting to see how long it is before Apple migrates the Mac Pro over to the new quad-core Xeon chip and makes an eight-core system publicly available. But unless you do work normally relegated to high-end workstations, perform massively multitasking workloads, or just want the bragging rights, eight cores is definitely overkill...at least for now. As more applications become available that support multithreading across multiple processing cores, the benefits of quad- and octo-cores will be realized.

System configurations:

Intel QX6700: 4 cores @ 2.66GHz, Windows XP
Windows XP Professional SP2; 2.66GHz Intel Core 2 Extreme QX6700; 2,048MB DDR2 SDRAM 800MHz; 256MB ATI Radeon X1900; 74GB Western Digital 10,000rpm SATA/150

Mac Pro: 4 cores @ 3.0GHz, Mac OS X
OS X 10.4.8; 2x 3.0GHz Intel Xeon 5160; 2,048MB DDR2 FB-SDRAM 667MHz; 512MB ATI Radeon X1900; 500GB Seagate 7,200rpm SATA/150

Mac Pro: 4 cores @ 3.0GHz, Windows XP
Windows XP Professional SP2; 2x 3.0GHz Intel Xeon 5160; 2.048MB DDR2 FB-SDRAM 667MHz; 512MB ATI Radeon X1900; 500GB Seagate 7,200rpm SATA/150

Mac Pro: 8 cores @ 2.66GHz, Mac OS X
OS X 10.4.8; 2x 2.66GHz Intel Xeon 5355; 2,048MB DDR2 FB-SDRAM 667MHz; 512MB ATI Radeon X1900; 500GB Seagate 7,200rpm SATA/150

Mac Pro: 8 cores @ 2.66GHz, Windows XP
Windows XP Professional SP2; 2x 2.66GHz Intel Xeon 5355; 2,048MB DDR2 FB-SDRAM 667MHz; 512MB ATI Radeon X1900; 500GB Seagate 7,200rpm SATA/150

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