Ever since hearing about the 'bus slewing' that is part of the G5, I have been interested in ho much it effects performance. If you don't know what bus slewing is, it is a way to conserve energy with your G5, while not giving up performance. To read more about it, check out the Apple Developer Note: Section on Power Management.

To determine the effect this setting has on computer performance, I ran a series tests. While only five applications were tested, the effect of bus slewing is pretty obvious.

Test System:
• G5 Dual 2 GHz
• 512 MB RAM
• Mac OS X 10.2.7
• ATI Radeon 9800
• 160 GB 7200 RPM SATA hard drive

Using XBench 1.1.2, I ran the whole test suite. The only test here that shows no difference is the disk test, as that has very little to do with CPU speed. Higher numbers (scores) are better.

Next, Quake 3 1.3.2 was run. All settings were maxed out, including sound, with no extra modifications to the configuration file. I tested 1024x768 and 1280x1024. The demo file I ran for this test was Demo 4. Higher numbers (frames per second) are better.

Then I went on to QuickTime's export function. I exported a 32.6 MB mpeg file to an mpeg 4 file at high quality. I emptied the trash (including the newly created file) each time so that the hard drive was in the same basic state. Lower numbers (seconds) are better.

On to CineBench 2003 version 8.1. This test renders and image using 1 CPU, and then 2 CPUs. It records the time and multi-CPU speedup. An interesting note, the multi-CPU speedup averaged 1.83x, which means a Dual 2GHz G5 with a multiprocessor aware app acts like a 3.66 GHz G5. Lower numbers (seconds) are better.

Finally, I ran the old standby for testing CPU speed, Altivec Fractal Carbon. This application renders fractals, and calculates the time required to do so. I left all settings at default except MaxCount, which I increased to 65536 to increase the time it took to render fractals. Lower numbers (seconds) are better.

There is almost no difference between the highest and automatic settings, showing that you do not lose performance at the automatic setting. However, at reduced setting, the number are significantly lower in all tests. If you are looking to keeping you G5 fast, along with saving a little money on your electricity bill, then the default automatic setting is the one to choose.