Overclocking: Frequency, Power, And Temperature Scaling
As you overclock, there's always a point when the CPU no longer runs stably and you have to stop pushing higher. The most well-known symptom of this is the blue screen of death (or BSOD). When you start seeing those, there are three possible remedies:
- Dial in a lower clock rate to regain stability.
- Modify other BIOS parameters and hope for a better outcome.
- Use a more effective cooling solution.
Cores versus Vcore: IHS In Place
What does increasing the Vcore setting do to frequency scaling? To answer this question, we used two different Core i9-7900X CPUs at various supply voltages in order to determine the highest clock rate able to complete a run of Cinebench R15. The voltage rose in 0.1V increments up through 1.2V, at which point we added a 1.25V setting for more granularity. Frequency measurements are reported with a precision of 25 MHz.
At their base frequency, both processors performed identically, completing the test at 4250 MHz. Increasing the Vcore by 0.1V bought us a maximum clock rate of 4575 MHz on our second sample, and 4600 MHz on the first one. That's a nice speed-up for a relatively small voltage increase. The next increment was similar, maintaining a 25 MHz gap between the CPUs for a total increase of 250 MHz on both of them. With 1.3V applied, the progression slowed. One Core i9 jumped 100 MHz and the other overclocked 150 MHz higher, widening the gap between them.
Adding Vcore is the primary means by which enthusiasts coax more frequency from their CPUs. But this approach is not without its limitations, as processors run hotter and their scaling slows. When the chip gets too hot, or when it requires a lot of extra voltage to hit slightly higher clock rates, it's time to stop.
Cores versus Vcore: Direct Die
This test zeroes in on our second Core i9 sample, comparing its frequency gains before delidding to direct-die cooling.
Even at the lowest voltage setting, direct-die cooling proved advantageous. However, the gap widened beyond 1.2V. The explanation is simple: prior to delidding, the processor ran hot, keeping the cores from utilizing the extra voltage available to them. The DDF helped it run cooler, allowing the CPU to stabilize at higher frequencies.
Enabling a 100 MHz-higher overclock and a temperature drop of 20°C, direct-die cooling is looking a lot more attractive.
Cache versus Vring
Now we're interested in exploring how the cache frequency scales as we increase the Vring setting.
Increments of 100 MHz explain the shape of our curve. Smaller steps would have yielded a smoother line. But the important take-away is that, for each 0.1V interval, we can hope for roughly 100 MHz of additional headroom.
Power Consumption and Temperature
Next, we looked at the effects of overclocking on power consumption and processor temperature by fixing the Vcore at 1.3V and varying the clock rate.
The first thing we noticed was the almost perfect relationship between temperature and power. It's also remarkable that the progression was practically linear. By increasing clock rate by 15.8%, power consumption jumped 14.8% and the temperature rose 13.5%.
Next, we fixed the frequency at 4250 MHz and raised the Vcore from 1.0 to 1.3V. The temperature still seemed tied to power consumption, but its increase was no longer linear. According to our measurements, pushing Vcore up 30% caused power consumption to rise by 63% and temperature to increase 55%.
Our data illustrating the impact of Vcore on the processor's temperature and power consumption gives enthusiasts one more reason to take it easy with that parameter during an overclock attempt.
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