iMac G5: Benefits of having matched RAM pairs; concerns over non-Apple-supplied memory

Reader Doug Eldred also sent us the following question: "Does anyone have any actual data for how much difference 'matched RAM' makes on the G5 iMac? Is this a mostly theoretical boost, or does it really provide a user-noticeable improvement?"

Doug is referring to Apple's documentation stating that:

If only one DIMM is installed, the memory bus is 64-bit. If two non-identical DIMMs are installed, there are two 64?bit memory buses. If two identical DIMMs are installed, the memory bus is 128-bit. Identical DIMM pairs have the same size and composition and provide the fastest and most efficient throughput.

Although we haven't yet received any real-world reports from readers, the German site PPCNUX provides benchmarks for a 1.8GHz iMac G5 using XBench 1.1.3. (Thanks, Simon-Pierre Rioux.) Specifically, the benchmarks compare an iMac G5 with two unmatched DIMMs totaling 1.25GB of memory to the same iMac with two matched 512MB DIMMs (for a total of 1GB). Even though the former system had more RAM, the "matched" system returned significantly better results:

System Fill:
Unmatched: 1625.25
Matched: 2231.49

System Copy:
Unmatched: 861.25
Matched: 1041.32

Stream Copy:
Unmatched: 1255.89
Matched: 1615.10

Stream Scale:
Unmatched: 1251.27
Matched: 1657.52

Stream Add:
Unmatched: 1423.78
Matched: 1781.15

Stream Triad:
Unmatched: 1441.01
Matched: 1794.59

Simon-Pierre also provides a download link to a software utility that will test whether or not the memory bus on an iMac G5 with two RAM chips is running in 64-bit or 128-bit mode. Interestingly, he claims that according to results provided by the utility, it's actually possible to achieve 128-bit mode with two DIMMs of different sizes, provided they're otherwise identical.

In order to achieve a matched pair configuration, however, some users decide to remove the included, Apple-provided RAM modules(s) and discard them through sale or otherwise.

This raises concern, however, of having a system with purely third-party RAM -- especially in light of a recent problem affecting single-processor Power Mac G5s where a firmware update (Power Mac G5 Uniprocessor Firmware Update 5.1.5f1) caused stability lapses and other issues that were resolved by removing third-party RAM.

In fact, in a Knowledge Base article for the revised version of the aforementioned firmware update states: "remove any third part RAM before installing this Firmware Update. After the Firmware Updater has verified that it has been completed correctly, you can re-install your third party RAM."

Without any Apple-supplied RAM, such a process is impossible.

As one MacFixIt reader writes:

"It occurs to me that this is a risky proposition, considering that the latest PowerMac patch says 'remove any third-party memory, install the patch, then reinstall the third-party memory.' If you no longer have anything BUT third-party memory..."

Resources