iMac G3 Rev A-D FSB bus overclock

[herd]

I'm sure that took a lot of time; thanks for posting your results! They look similar to my testing with G4 CPUs. Memory bandwidth and cache seem to make more of a difference with big messy software like compilers, browsers, emulators, etc. Dual CPUs also like cache/memory speed. I saw big differences compiling packages under linux with gcc 15 by changing cache or bus configuration.

 

[indibil]

Here are my benchmarking results for the 100MHz bus overclock, compared to stock.

CPU was set to 300MHz for both the 66MHz and 100MHz configurations.

Each test was performed three times and an average score recorded.

Task	Results	Increase
Xbench benchmark	7.78 --> 8.48	8.3%
iTunes import	52s saved on 1333s	3.9%
Handbrake import	106s saved on 1715s	6.2%
System Info benchmark	799 --> 800	0.1%
Photoshop actions	15s saved on 279s	5.4%
Disk Copy checksum	1s saved on 61s	1.6%
Unstuff large file	10s saved on 353s	2.8%
Unstuff large file (RAM Disk)	13s saved on 326s	4.0%
Unreal Tournament	11.36 --> 12.13 FPS	6.8%
Gauge Pro benchmark	65.5 --> 89.3 MB/s	36.3%

The difference between a 66MHz and 100MHz bus on this machine is marginal (4-5% real world - excluding Gauge Pro) and hardly noticeable in real world use. For a 50% increase in bus speed, I do find this surprising. But an explanation can be found in the G3's large 1MB L2 backside cache – when disabled the bus speed then becomes a lot more relevant:

Configuration	System Bus	Avg. Real World Gain	Bottleneck
1MB L2	66 --> 100MHz	~4.3%	CPU Clock Speed (300MHz)
L2 Disabled	66 --> 100MHz	~14.0%	RAM Latency/Bus Bandwidth

Apple likely had a greater rationale for going to a 100MHz bus on the Blue and White G3; with multiple high speed PCI cards, bus speed would matter quite a lot. On an iMac like this one, only RAM latency is realistically affected and this is mooted by the backside cache.

Great work!!!

Were you able to compare the performance difference in operations like booting up, copying large files, or copying several small files?

In these cases, would the increase be more noticeable? Daily use.