We have not seen anything unexpected in our today’s tests. Like with Intel platforms, the frequency and timings of DDR3 SDRAM may have but a small effect on performance of the Socket AM3 platform. A modern processor has enough cache memory to smooth out any latency that occurs on the path that data takes towards the execution cores. Working in its nominal mode, our Socket AM3 system with a newest Phenom II X6 processor (which has 6 MB of L3 cache) would only get an average 2-3% faster at higher memory settings. The performance may increase up to 5% in some applications that operate with huge amounts of data but this doesn’t change the overall picture. When the system components were not overclocked, there was almost no difference as to what memory was installed: DDR3-1067 or DDR3-1333 or DDR3-1600. You only have to make sure that it works in dual-channel mode.
This rather boring situation can be enlivened somewhat by overclocking. AMD processors allow to overclock the memory controller and L3 cache separately from the execution cores. This overclocking is quite rewarding. Rated for 2.0 GHz, the Uncore part of the CPU can be clocked at much higher frequencies, which leads to a performance growth up to 5%. You shouldn’t neglect this if you really care about your computer’s memory performance.
When you increase the base clock rate, the frequencies of the execution cores and the Uncore part of the CPU grow up and higher DDR3 SDRAM frequencies become possible. Coupled with the increased gaps between the available memory frequencies, this makes the effect of different memory frequencies and timings easier to see. For example, the difference between the slowest and fastest memory mode at a base clock rate of 250 MHz amounts to an average 5%, reaching 8-9% in some applications. And we even could not start our system up using the fastest DDR3-2000 mode, which requires additional comments.
AMD and mainboard makers have assured users that new mainboards based on the AMD 890FX chipset support DDR3-2000 mode if used together with a Phenom II X6 series processor. However, this is just a marketing claim that is not well grounded. The sentence lacks an important reservation like “if the planets of the Solar system line up in a particular order” or something. During our tests we could only reach memory frequencies of 1800-1900 MHz which is the top limit you can expect from good hardware. Higher results are only possible if you carefully pick up the most overclocker-friendly samples of mainboards and, most importantly, processors.
Thus, most AMD users cannot benefit from high-speed memory available in shops today. They should not care to purchase overclocker-friendly modules with a rated frequency of higher than DDR3-1866 as they won’t be able to utilize the speed potential of such memory fully.