Exclusive: ZDNet overclocking tool enhances performance of Mac Pro

Why does ZDNet Clock run on Mac Pro computers only?
Different models use different clock chips. These chips must be programmed in a totally different manner. ZDNet started with Mac Pro since they are the preferred computers of users who demand maximum power. Feedback from our readers shows us that there is much interest in support for more models. MacBook owners should be aware that they will not achieve a great performance increase when overclocking. Running a notebook computer under full load will push the temperature to the limit after a couple of minutes, even if not overclocked. This causes the CPU to throttle. Overclocking a notebook computer results in better performance for a few minutes only.

Is there a chance that my computer will not boot after running ZDNet clock?
No, ZDNet Clock does not load a kernel extension (kext) at boot time. After powering off, the system boots at stock speed.

Will ZDNet Clock shorten my computer's life?
Hardly at all, since ZDNet Clock does not increase the voltage. We estimate that the computer's life expectancy may be shortended from 20 years to 19.5 years. Core 2-based CPUs at stock speed run well below their theoretical limits, and one can generally expect a long lifespan.

Are there any risks involved in overclocking by a great deal?
As with any system crash, you lose unsaved files. Since HFS+ is a pretty robust filesystem, it recovers well at the next reboot.

Benchmarks, for example XBench, Cinebench and Geekbench, do not run faster after overclocking.
Yes, they do. As stated on page 3, Mac OS X uses the bus clock as a real time source. This causes the system clock to run faster, so benchmarks will show the same results as without overclocking. Use a stopwatch to see the performance increase.

Why does the system clock run faster? Is there anything I can do?
This is due to the fact that Apple adjusts the real time clock to bus clock ratio at boot time only. On newer Mac Pros (early 2008), simply do a warm reboot. There is currently no solution for older Mac Pros.

Apple did something wrong? This is impossible.
If you use the bus clock as a real time source, this works as long you do not change the bus clock's speed. However, especially on notebooks, the bus clock does not run at constant speed, even without overclocking. Under full load, upon reaching a certain threshold temperature, notebook processors will leave out bus cycles to cool down. This leads to known problems with USB and Firewire audio devices. Audio playback may start stuttering. The same problems occur when you overclock a Mac Pro.

On notebook computers, the problem disappears when then computer runs idle for a while and has some time to cool down. On a Mac Pro, you have to revert to stock speed. An acceptable workaround is to overclock only when you specifically benefit from it, for instance while encoding or rendering.

If you read between the lines of some of the answers in the Apple developer support forum, you will realize that Apple is obviously aware of these problems. There seem to be some integration issues between the Mach part and the BSD part of the kernel. Most likely, they are working on it.

My system crashes when I overclock by only 5 MHz. What is the cause?
Overclocking means a speed increase of the CPU, front side bus and main memory. All those components must be able to operate safely at a higher speed. If Apple install a CPU specified to run at 2.80 GHz, they cannot be blamed if it does not run at 2.81 GHz.

In ZDNet's experience, RAM is the most likely component to fail when overclocking. Pre-installed Apple memory is generally of very good quality. Cheaper RAM from Transcend or Kingston performs even better. Many other brands, however, do not meet ZDNet's expectations. Corsair even refused to supply ZDNet with samples. They state that their memory modules are not designed for overclocking.

One should also be aware that Transcend, Kingston, Corsair, OWC, iRAM and CSX are just brand names. These companies get supplies from a variety of sources. Quality may vary from batch to batch.