What Is Better: BCLK Frequency vs. Multiplier
When Core i7-875K and Core i5-655K start selling, most LGA1156 overclocker systems will allow processor overclocking by raising the clock generator frequency or changing the clock frequency multiplier (we are only talking about systems using non-extreme cooling methods). Of course, it is quite logical to wonder: which overclocking approach would be preferable?
We decided to clarify this matter and test Core i7-875K processor working at 4.0 GHz frequency in two situations: when its frequency is achieved by pushing the BCLK clock to 200 MHz and when BCLK stays at its default frequency of 133 MHz, but the multiplier is increased instead. I have to say that during overclocking by raising the clock generator frequency we even lowered the multiplier a little bit (to 20x). It can be done to any system (even the one using a CPU with locked multipliers) in order to match the memory frequency accordingly. As a result we ended up testing two similar systems for our comparison:
- Core i7-875K processor at 4.0 GHz = 20 x 200 MHz, DDR3-1600 memory (9-9-9-24-1T)
- Core i7-875K processor at 4.0 GHz = 30 x 133 MHz, DDR3-1600 memory (9-9-9-24-1T)
You can see from the screenshots above that different approaches to overclocking result in different Uncore and QPI bus frequencies. When BCLK is increased beyond the default 133 MHz, the frequency of these two knots increases proportionally. These are the factors that determine the performance differences:
The test results show that the difference in overclocking approaches does affect the performance. And overclocking by raising the BCLK frequency proves to be more rewarding than overclocking using the processor clock frequency multiplier. In fact, it is quite logical, keeping in mind that the clock generator frequency is tied to the frequency of the QPI bus, memory controller and L3 cache. The performance difference is especially dramatic in a synthetic test measuring the memory and L3 cache performance. However, even in real applications overclocking with BCLK frequency produces about 1-2% better performance. Obviously I can’t claim that it is an impressive advantage, but dedicated computer enthusiasts fine-tuning their systems to the utmost extent will definitely appreciate this improvement.