Articles: CPU

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Although the first two processors for LGA 2011 systems including Core i7-3960X and Core i7-3930K have unlocked clock frequency multipliers, which automatically eliminate all overclocking limitations, it is also very interesting to see how far they can be overclocked by raising the clock generation frequency (BCLK). The problem is that with the launch of processors on Sandy Bridge microarchitecture Intel started using the same unified clock frequency generator for all system frequencies. Therefore, the increase on BCLK clock in LGA 1155 platform immediately caused stability issues, because the system was unable to function normally at increased frequencies of the DMI and PCIe busses as well as SATA and USB controllers. So, the only way LGA 1155 users could overclock their systems was to purchase a K-series processor with an unlocked multiplier.

LGA 2011 platform doesn’t have this problem, but in Q1 2012 we should see Core i7-3820 model, which will no longer boast unlimited adjustment of the clock frequency multiplier. This is when the BCLK overclocking approach will be brought up again.

Luckily, Intel has a positive answer to this question. No, independent clocking of the processor and other system components is long gone, but the developers came up with another solution. On the one hand, it involves only one clock frequency generator, and on the other, it makes it possible to overclock LGA 2011 processors ”by bus”. The main idea behind this solution is the introduction of an additional frequency multiplier, which is applied to the BCLK frequency right before it is fed to the CPU. This multiplier may be set at 1.0x, 1.25x or 1.66x, which will be equivalent to 100, 125 and 166 MHz base clock with all other system frequencies remaining at their nominal values.

As a result, LGA 2011 systems acquire much greater potential than LGA 1155 and allow more room for “bus maneuvering”. If we take 100, 125 and 166 MHz BCLK frequencies, at which all system components (except the processor) work in nominal mode, and increase them by 10-15%, which usually doesn’t cause any problems, we will end up with a pretty wide range of processor base clocks with small “dead” zones around 112 and 143 MHz. Although the mainboard makers informed us that not all processors will be able to handle 166 MHz base generator clock and we may need to carefully select the most quality units before attempting it. On the other hand, at 125 MHz bus frequency there shouldn’t be any problems at all, so 25% overclocking without any multiplier adjustments will always be available.

But, let’s move from theoretical discussion to practice. Core i7-3960X and Core i7-3930K processors do not need any BCLK adjustments, as they can be easily overclocked by increasing their clock frequency multiplier. Moreover, they also allow changing the multiplier used for the system memory frequency, which in LGA 2011 systems may fall between DDR3-1067 and DDR3-2666 with 266 MHz increment.

However, despite the seeming simplicity of the overclocking procedure, the new processors did upset us a little bit. As we found out Intel lowered Tjunction Max temperature for their Sandy Bridge-E processors. Upon reaching this maximum temperature thermal throttling kicks in and clock frequency drops for a short while. For our specific CPUs these temperature thresholds were set at 86°C and 91°C respectively. The Extreme Edition Core i7-3960X could heat up to 91°C, while Core i7-3930K enbaled thermal throttling at 86°C. This is a very serious limitation of the overclocking freedom, especially taking into account that LGA 1155 and LGA 1366 processors can work perfectly fine at up to 98°C and 101°C respectively. As a result, you need much more efficient cooling for successful LGA 2011 overclocking . Moreover, highly efficient cooling will be necessary for yet another reason: the heat dissipation of the Sandy Bridge-E processors is considerably higher than that of not only quad-core Sandy Bridge CPUs, but even six-core Gulftown processors.

As a result, it may be challenging to overclock Sandy Bridge-E processors with traditional cooling systems to 4GHz+ frequencies. In most cases it turns into an intense struggle with threshold temperatures and an exhausting search for the optimal frequency and Vcore combinations, which won’t push the CPU into throttling and interfere with system stability.

During our experiments we managed to get our Core i7-3960X and Core i7-3930K processors to work stably only at 4.5 GHz. To maintain system stability we had to increase the voltage to 1.38 V for Core i7-3930K and to 1.435 V for Core i7-3960X. At the same time we enabled Load-Line Calibration. We tested system stability using LinX 0.4.6 with an updated Linpack library supporting AVX instructions.

Core i7-3960X Extreme Edition overclocked to 4.5 GHz

Core i7-3930K overclocked to 4.5 GHz

Note that the maximum core temperatures are just a little lower than the critical value, especially when we overclock the top model, even though it works at a lower Vcore. But luckily, the throttling doesn’t kick in here, which we can clearly see from the steady and constant GFlops value in our LinX utility.

I have to stress that the above described overclocking was possible only due to highly efficient NZXT Havik 140 super-cooler. Unfortunately, Intel’s liquid-cooling RTS2011LC system didn’t cope with cooling our overclocked processors. The best we could do with this system was 4.3 GHz, and after that we have to give it up.

So, six-core LGA 2011 Core i7 processors overclocked to lower frequencies than what quad-core LGA 1155 processors are capable of. So, it is quite possible that the new platform will be less attractive for some overclockers than the old LGA 1155, where acceptable processor temperatures are not so strictly limited. Especially since with the upcoming launch of the seventh-series chipset for this socket, too, Intel promises to allow “by bus” overclocking also in the “junior” platform.l

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