For our tests we used the following testbed:
- CPU: Core i5-655K (Clarkdale, 2 cores/4 threads, 3.2 GHz, 4 MB L3);
- Mainboard: ASUS P7P55D Premium (LGA1156, Intel P55 Express);
- Memory: 2 x 2 GB, DDR3 SDRAM, GeIL EVO ONE PC3-17000 (GE34GB2133C9DC);
- Graphics card: ATI Radeon HD 5870;
- HDD: Western Digital VelociRaptor WD3000HLFS;
- CPU cooler: Thermalright Ultra-120 eXtreme with Enermax Everest fan;
- PSU: Tagan TG880-U33II (880 W);
- Operating system: Microsoft Windows 7 Ultimate x64;
- Intel Chipset Driver 126.96.36.1995;
- ATI Catalyst 10.5 Display Driver.
Test 1: Nominal Mode
The first part of our test session was devoted to the work of our system in its nominal mode, when none of the system components were overclocked. Only the multiplier for the memory frequency and memory timings were changed. I have to say that during this test we tried to emulate the most typical operational conditions for our platforms, so we decided not to deactivate any processor technologies. Hyper-Threading, Turbo Mode and Enhanced Intel SpeedStep worked as usual: the system saw our Core i5-655K processor as a quad-core one, and its clock frequency increased to 3.33 GHz or 3.46 GHz under computational load of different intensity.
At first we were going to test Core i5-655K in all possible modes, which are in this case more numerous than by regular LGA1156 processor. The regular Core i5 CPUs can only clock the memory as DDR3-800, DDR3-1066 or DDR3-1333, but the overclocker Core i5-655K also supports DDR3-1600, DDR3-1866 and DDR3-2133 modes. At least, this is the conclusion we managed to draw after checking out the settings available in the mainboard BIOS with this processor. However, practical experiments showed that not all the memory configurations are operational. In particular, as we have already mentioned above in the description of our DDR3-2133 memory, Core i5-655K failed to work stably with the memory frequency at 2133 MHz. Therefore, we had to eliminate the DDR3-2133 mode from our tests. Another problem emerged in DDR3-1600 mode. In this mode the memory remained stable only with less aggressive timings set to 9-9-9-27 or 8-8-8-24. When we set the timings to 7-7-7-21, the system would freeze on boot-up, although GeIL GE34GB2133C9DC definitely supports this operational mode according to the spec. In other words, the Clarkdale memory controller is not as simple as it seems at first glance, so it is obviously very strange that the regular representatives of the Clarkdale family do not have any coefficients that could allow clocking the memory at frequencies past DDR3-1333.
We used Cachemem benchmark in Lavalys Everest utility to test the memory subsystem bandwidth and latency.
Things are very interesting here. On the one hand, memory frequency increase and lower timings logically lead to lowering of the overall practical latency. But on the other hand, if we look at the performance level during reads from the memory subsystem (and it is one of the most important practical parameters), its growth will not be really noticeable. It seems that with memory working at speeds past DDR3-1333 the bandwidth along the path between the processor computational cores and memory is artificially limited by some obstacle. So, it would be quite logical to assume that this obstacle is none other but the internal bus connecting the processor die with the die containing the memory controller itself. In other words, using high-speed memory with Clarkdale processors without any overclocking by raising the base clock frequency won’t do much good.
Of course, the benchmark results reflect this observation clearly:
Although fast DDR3 SDRAM only provides lower memory subsystem latency, barely affecting the actual bandwidth increase, we can see that the performance increases in a number of tests when we install faster memory modules into our testbed. On average, the use of DDR3-1866 instead of DDR3-1333 delivers about 3% performance boost. This is exactly what Intel takes away from us by telling us not to use faster memory than 1333 MHz with their Core i5 and Core i3 processors. In our opinion, this is a pretty ephemeral increase in actual speed, that gives us good reasons to doubt the need for high-speed DDR3 SDRAM in non-overclocked LGA1156 systems. Sad as it might seem, but the memory controller in Clarkdale based systems is no match for the memory controller in Lynnfield under no circumstances, so the increase in the memory frequency does absolutely nothing in this case.