HANDS ON GUIDE

Benchmarking And Overclocking

During IDF Fall 2008, various speakers dropped hints and clues as to how Nehalem will behave when overclocking. We’ve collated these, so you can get a head start of those benchmark leaderboards as soon as your Core i7 arrives.

BENCHMARKING AND OVERCLOCKING

Overclocking a Core i7 processor will be trickier than merely pushing the Front Side Bus as high as it’ll go - and not only because Core i7 doesn't use a Front Side Bus. Instead, Core i7 has an internal clock running at 133MHz that determines three important system frequencies. The first of these is the speed of the QuickPath bus that connects the CPU to the rest of the system. It also affects the QuickPath bus between the onboard memory controller and the DDR3 memory. It can also affect the speed of the memory controller itself. With the latter two, we expect many motherboards will provide options to adjust these speeds in the BIOS.

It also seems that Nehalem-based processors will have three internal voltages. A good overlocking motherboard will need to provide fine degrees of voltage control as Nehalem-based CPUs will have low vcore values. Overvolting in 0.1V increments will be too crude to get the most from your chip.

DON’T TOUCH THE 133MHz CLOCK GEN

Intel’s Senior Performance Analyst, Francois Piednoel, who probably knows more about overclocking Nehalem than anyone else in the world, gave us some strong advice. ‘Don’t change the internal clock of Nehalem,’ he says, ‘we don’t know what it’s doing when you do. Seriously.’

On the plus side, Piednoel did say that overclocking the CPU-to-system QuickPath bus yields great results. A higher frequency (upping the bandwidth from an already astounding 25.6GB/sec) gives a good deal more performance: ‘it’s almost like adding another graphics card,’ claims Piednoel. We expect that a little tinkering in the BIOS of an X58 motherboard with a good Northbridge heatsink should yield a cheap and easy speed boost.

PERFORMANCE vs HEAT

Keeping excess heat output under control has long been the bane of overclockers, and the problem will become even worse with Core i7. The Nehalem architecture could be described as self-aware (though thankfully not in a futuristic killer-robot kind of way) as the sensors in each of its processing cores are able to decrease or increase the frequency of these cores on the fly.

Nehalem's ability to auto-manage these frequency fluctuations is down to TDP, or Thermal Design Power. During IDF we were repeatedly told that Nehalem wouldn’t exceed the previous 130W TDP of Core 2 CPUs, but that it would dynamically manage its workload and the frequencies of its cores to maximise performance within this power budget.

While Turbo Mode can only increase the frequency of any one core by 266MHz, most overclockers will aim higher. To do this, you’ll need to tell the BIOS of your motherboard what the TDP rating of your CPU cooler is (or you can set this option to infinity).

Thankfully, you’ll be able to turn Turbo Mode off. With a big overclock applied, Nehalem will still shut down unused processing cores in an attempt to overclock those being used. If you’ve gone for an extremely high overclock, you’ll find that Turbo Mode might push your silicon beyond it limits and your system will BSOD.

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