During the very first discussion of the new dual-core LGA1156 processors from Intel that belong to the Clarkdale family, we pointed out a few unpleasant peculiarities of its memory controller. These processors work with the memory very slowly, as you can see from comparing the results of memory subsystem bandwidth and latency tests in systems built around dual-core Core i5 and quad-core Core i7. For example, Everest Memory Benchmark launched on a new dual-core Clarkdale and a quad-core Lynnfield working at the same clock frequency produces the following results:
Clarkdale 2.8 GHz
Memory Read, MB/s
Memory Write, MB/s
Memory Copy, MB/s
Memory Latency, ns
Although the dual-core processor was manufactured with more advanced 32 nm technological process and came out later than its quad-core counterpart, it is lagging behind the latter quite substantially. However, this lag is no unexplainable phenomenon for us. The thing is that unlike quad-core Lynnfield processors, dual-core Clarkdale CPUs do not have monolithic structure, but consist of two semiconductor dies put inside one packaging. At the same time, the memory controller is located inside a separate die than the computational cores. That is why the memory subsystem works slower, since there is now an additional QPI bus on the stretch between the processor and the memory, which helps the dies inside Clarkdale to communicate with one another.
For this particular reason the peculiarities of memory subsystem functioning in systems built around dual-core Core i3 and Core i5 processors deserve a separate study. It obviously makes no sense to generalize the results obtained earlier in LGA1156 systems equipped with quad-core CPUs. Therefore, we decided to carry out an individual test session that will reveal the influence of memory clock frequency and latencies on the performance of Clarkdale based systems.
This topic appears even more acute due to the fact that we got out hands on an overclocker Core i5-655K processor with an unlocked multiplier (this CPU is also available in retail already). It not only allows adjusting the multiplier to achieve the desired clock speed, but also offers access to a wide range of memory operation modes. While regular Clarkdale processors only supports DDR3-1333 SDRAM in nominal mode, the processor model with an unlocked multiplier also allows clocking the memory faster, namely as DDR3-1600, DDR3-1866 and DDR3-2133.
I hope that the ability of Core i5-655K processor to support higher-speed memory modes will at least partially make up for the slow memory controller, because in most cases it is the memory controller that becomes the bottleneck of Clarkdale microarchitecture preventing these processors from unveiling their true performance potential.