However, the primary conclusion that we can draw from the results of our today’s test session is that the performance of dual-core LGA1156 processors doesn’t seriously depend on the memory speed. And this is something that is not so unique for Clarkdale processors: we have already talked about the minimal effect of the memory speed on performance many times before.
But there is one peculiarity in this case. Although Clarkdale processors formally have an integrated memory controller, in reality the controller is inside an individual semiconductor die that is connected with the processor die via QPI bus. This additional bus becomes a bottleneck that causes the performance to increase minimally when the memory is faster than DDR3-1333. However, DDR3-1600 or DDR3-1866 SDRAM without the overclocked BCLK frequency can only be used with Core i5-655K CPU with an unlocked frequency multiplier, which is not really that widely spread. Regular dual-core Core i5 or Core i3 processors do not allow clocking the memory at anything higher than 1333 MHz in nominal mode.
During overclocking by raising the BCLK frequency, the frequency of the notorious QPI bus also increases, so there are no more problems with high-speed memory in systems with the overclocked Clarkdale processor. Instead, we discover another peculiarity: it is better to stay away from 8x multiplier for the memory, because the actual memory subsystem bandwidth for some reason educes substantially in this case. Therefore, it is best to use either the smaller multiplier of 6x with the minimal timings, or the maximum multiplier of 10x to achieve maximum performance.
In the best case scenario, you can gain as much as 8-10% in extra performance by playing with the memory subsystem parameters. It is up to you to decide if that is enough to justify the investment into overclocker memory. But it definitely makes no sense to buy anything faster than DDR3-2000 for a system with a dual-core LGA1156 processor. Even if you have a Core i5-655K CPU with an unlocked frequency multiplier (not to mention regular Clarkdale CPUs), you won’t be able to use super-fast memory unless you have extreme cooling solutions.
Here I would only like to add that despite all tricks, we could get the controller of our Clarkdale CPU just a little bit closer to its counterpart in Lynnfield processors. We had to overclock all busses in our Clarkdale based system by 50% and use high-speed overclocker DDR3-2000 SDRAM in order to get our memory subsystem as fast as that in Lynnfield based configurations with DDR3-1333 SDRAM. So, more expensive quad-core LGA1156 processors are indisputably faster than their dual-core brothers not only when it comes to computational power, but also to work with the memory subsystem.