Apple Makes the Switch: iMac G5 vs. iMac Core Duo

IBM vs. Intel - Performance per Watt

At Steve Jobs' WWDC 2005 Keynote, he presented this slide to the curious crowd:

The idea was that IBM couldn't deliver the performance per watt that Intel could over the coming years, and that was one of the primary reasons for the transition away from the PowerPC architecture.

While we can't test future IBM and Intel CPUs, we can definitely compare power consumption and performance per watt today and find out if Intel can at least start delivering on Apple's claims.

I started out by looking at power consumption of the iMac systems as a whole, measured at the power outlet in Watts. The systems are configured as close to identical as possible; although the Core Duo iMac has a slight advantage in its 90nm X1600 GPU compared to the iMac G5's 0.13-micron X600 GPU, I didn't focus on any GPU intensive tests so the difference should be minimized. There are obvious memory differences as well. While both systems have 512MB of memory, the Intel system uses DDR2-667 and the G5 system uses DDR2-533. Then there are the CPU and chipset differences, but those are the ones that we want to measure anyways, so they have to be there.

The first comparison is system power consumption at idle. This is with both identically configured systems being freshly booted and sitting on the desktop with no disk accesses or anything. Note that the systems weren't left alone long enough for the hard drive to spin down yet:

The Intel based iMac consumes about 2/3 of the power of the iMac G5 - impressive and pretty much expected given what we've seen of the Core Duo in the past. The G5 is a solid competitor here, but the Core Duo is in a completely different league of power consumption.

Under load, the difference in power consumption is just as pronounced - with the iMac G5 coming in at 96W while the Intel based iMac is at 62W. Once again, we are looking at approximately 2/3 the system level power consumption from the Intel based iMac.

I also looked at power consumption during my H.264 encode test, which produced results similar to the iTunes encoding test above:

The iMac Core Solo 1.83GHz was simulated by disabling one of the Core Duo's cores

But now let's look at performance per watt and see where we stand today. I took the Quicktime H.264 encode time and converted it into frames encoded per second, then divided that by the peak power consumption values for the entire system. Note that during the Quicktime and iTunes encoding tests, the peak power consumption value and the average power consumption of the system are basically identical, since the CPU remains at close to 100% utilization for the entire duration of the test.

The end result is this graph of Frames per Second/Watt:

The iMac Core Solo 1.83GHz was simulated by disabling one of the Core Duo's cores

It looks like with the Core Duo, Apple does have better performance per watt than with the G5. The Core Duo based iMac offers about 90% higher performance per watt than the iMac G5. Even when looking at single core performance, the performance per watt is higher, although not nearly by as great of a margin at 19.7%. It is worth noting that Intel's performance-per-watt advantage would be severely diminished if we were comparing to a 65nm G5 processor, which as of yet does not exist. By the time that IBM is shipping 65nm desktop processors in volume, I'd expect Conroe/Merom to be out and both of those cores are supposed to offer greater performance (both directly and on a performance per watt basis) than Core Duo, so it is possible for Intel's advantage to continue on into the future.

We are only talking about system power usage here, so if we were to truly isolate just the CPU power, the difference should be even larger, assuming that the rest of the systems consume a relatively equal amount of power in both configurations. You should also keep in mind that the Quicktime encode test is actually one of the Core Duo's weakest tests, so the actual performance per watt advantage may be even higher in tests where the Core Duo has a larger performance advantage over the G5. This is just a good test to show power consumption, since it is fairly CPU bound and consistent in its CPU load.