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DDR 400 Memory Speed Tests on the Power Mac G5

Originally posted 12/10/03 by rob-ART morgan, mad scientist HYPOTHESIS: If you buy more expensive memory with lower latency rating, your G5 will go faster. When I purchased the Dual G5/2.0 for the Bare Feats lab, I wanted it to have the best performing drives and memory. I researched the various memory options. Windows PCs sites have done some interesting memory testing. I decided to order two 1GB kits: Kingston Hyper-X DDR400 CAS2 (2-2-2-6 rating) and OCZ Platnium Enhanced CAS2 (2-2-3-5 rating). They cost about $250 per kit back in September. The Hyper-X wouldn't let the G5 boot. The OCZ let the G5 boot, but, according to Xbench's memory test, it was no faster than the CAS3 factory memory. So that was a waste of money. I traded the Kingston Hyper-X memory for some cheaper CAS 3 memory. The G5 boots fine and it the G5 runs just as fast with it as it does with the pricey OCZ. A few weeks ago, I received four 1GB DDR400 (CAS3) modules from Other World Computing to test. Though they were being tested to see how best to optimize memory hungry apps like Photoshop, I was also interested to see if they affected the speed of the G5. When four more 1GB DDR400 (CAS2.5) modules arrived from Trans International, I could see I needed to have a shootout between the 1GB kits. (heh, heh, heh) I mentioned Xbench before because it contains 7 memory speed tests. The three System tests are Allocate, Fill, and Copy. The four STREAM tests are Copy, Scale, Add and Triad. I made two runs with the OWC and TransIntl memory. To my surprise, the TransIntl memory was faster. Overall Xbench rating was 350 compared to 304 on the OWC. When I looked at the breakdown, the biggest difference was in the FILL test. TransIntl's memory was FILLing at the rate of 3GB/s. OWC's was FILLing at the rate of 1.8GB/s. Whoa. Big difference. Before I could do any more testing on the TransIntl memory, I had to send it back. But they sent me another four 1GB modules of DDR400, only this time the memory was the more expensive Micron ($200 more per stick than the previous Samsung variety). I ran the Xbench memory tests again. This time the OWC memory scored a FILL rate of 3GB/s. "Hey, what's going on?" I made four more runs. The speed dropped to as low as 2.3GB/s. "Huh?" I put the new TransIntl memory in. It scored a FILL rate of 2.9GB/s in the first run but dropped to as low as 2.3GB/s on the next three runs. Very strange. I decided to abandon Xbench and run 5 cpu/memory intensive real world applications. I got identical results for both the OWC and TransIntl memory.   BEST THEORY: The strange memory benchmark behavior (dropping fill rate) is the G5 throttling down the memory to reach a speed of its liking. BUYING ADVICE: Get the cheapest DDR 400 CAS3 memory you can find, as long as it works in your G5. Buying expensive memory with low latency ratings and heat spreaders made of exotic metals will NOT make your G5 go faster. (Normally I like to throw up some interesting graphs, but as this point, I have nothing interesting to graph.)   LINKS OF INTEREST XLR8YourMac tested a G5 with different memory configurations from 512MB to 8GB. You will note there was very little advantage to putting more than 4GB because Photoshop limits the memory cache setting to 2GB or less. Hopefully, they will change that so that high end Photoshop users can take advantage of the 8GB capacity of the G5. Of course, the full 8GB comes in handy if you are loading up multiple applications. Also, the more memory you have, the less disk "scratching" the OS has to do. It also gets used for disk caching.   WHERE TO BUY Other World Computing has 2GB DDR memory kits as well as 1GB DDR memory kits for the various models of G5. (You must install them in pairs. A 2GB kit consists of two 1GB sticks.) Trans International has 1GB modules for all the models of G5. They also have them for the Aluminum PowerBook G4s and the iBook G4. Right now 1GB DDR400 modules cost twice as much as two 512MB DDR400 modules. If you are going for just 4GB, you can buy 8 sticks of 512MB DDR400 for as little as $552. If you go with 4 of the 1GB sticks, it will cost you at least $1176.