When building any modern computer, the choice of which memory to use is a major consideration. After all, why spend a mint on the latest CPU and motherboard, only to slow it down a bit with anything but the best memory available? This was definitely a big consideration when we began work on our Ultimate Linux Workstation. So, the question before us was, “Which memory should we buy and would the expense of the lower latency memory be worth it?” To answer this question, we went in search of the three standard CAS latencies that you will find for DDR memory and we put them to the test on our new Opteron testbed.
For this testing we went out and found memory specifically designed to run at the settings we wanted to test. This made the testing more real world, since these modules represented certain price points.
For the CAS 2 and CAS 3 memory, we turned to Corsair and their XMS and Ultra-Stable server memory respectively. For CAS 2.5, we called up Kingston for some of their HyperX registered ECC modules. Both companies were more than happy to participate in our testing. This also gave us an opportunity to test out these memory modules for compatibility on the new Tyan Thunder K8WE motherboard.
First of all, were all of the memory modules stable? Yes. All modules made it through all stress testing without error. For our testing we ran all modules with ECC enabled. This is a requirement for our CAS 2 modules, so we thought it was only fair to run ECC on all modules. We've found through internal testing that ECC does not appear to measurably hurt performance. The only hitch we found in testing is that Kingston's modules did not SPD (Serial Presence Detect) at the advertised timings. This was not a huge issue though since we were able to set these manually. Both sets of Corsair's modules set themselves to the timings expected.
Now let's show you our setup: Link  to setup, tests and results.