Overclocking by the bus!We started by overclocking the processor, which differs from the LGA 1155 platform somewhat. In effect, overclocking by the bus is back!
At first, DMICLK speed is however locked at around 105 MHz as on LGA 1155. This clock is used as a basis for the other buses but Intel enables the application of a multiplier for the clock used by the processor. Two multipliers at x1.25 and x1.67 are available.
In practice we were only able to get the x1.25 mode to work, with a maximum bus clock of 131.25 MHz (105x1.25). The x1.67 mode wouldn't work, even at 150.3 MHz (90x1.67).
Is it worth having this option all the same? Of course, it will enable overclocking of the forthcoming Core i7-3820, but the 3930K and 3960X are unlocked and the gain is virtually nil as shown by these differences obtained at 4.5 GHz between a 45*100 setting and another at 36*125:
- 3ds Studio Max 2011: 0.23%
- Visual Studio 2011: 0.89%
- 7-zip: 0.82%
- Lightroom: 0%
- Fritz Chess Benchmark: 0.19%
In fact the gain comes more from the memory setting, with a bus at 100 MHz using DDR3-1600 and a bus at 125 MHz using DDR3-1666. In effect, the following basic ratios are available:
- 100 MHz: DDR3-800, 1066, 1333, 1600, 1866, 2133, 2400
- 125 MHz: DDR3-1000, 1333, 1666, 2000, 2333, 2666, 3000
As you can see, there’s not really any point in overclocking by the bus when it comes to practical performance and any advantage given only shows up in extreme benchmarks.
What about overclocking the processor itself? In Prime95, our CPU already consumes 136.8 Watts at the ATX12V, with the internal processor reading showing as 129 Watts in HWMonitor.
With Turbo, the processor then clocked at around 3.6 GHz. To start with we tried to work out what the minimal voltage possible was for a clock of 3.6 GHz in Prime95. We managed to drop down to 1.1V in the bios, without being able to tell you what the actual voltage was as the applications weren‘t able to read the vCore on the DX79SI. Energy consumption was down 36W at the ATX12V in comparison with the default configuration.
Next we started overclocking the CPU in 200 MHz jumps and we managed 3.8 GHz at 1.15V, 4 GHz at 1.25V, 4.2 GHz at 1.3V and 4.4 GHz at 1.35V. At 4.4 GHz energy consumption at the ATX12V increased to 201.6 Watts, or 47.6% higher than the initial configuration.
We then changed to jumps of 100 MHz, stabilised by voltage increases of 0.05V and stopped at 4.6 GHz and 1.45V. Here we took a reading of 256.8 Watts at the ATX12V or 87.7% more than the initial configuration. We couldn’t stablise the CPU at 4.7 GHz and 1.5V and energy consumption then reached 290 Watts.
While overclocking to this level was a decent performance for our Core i7-3960X, it wasn’t anything exceptional.