AMD FX Series processors paired with the 9 Series chipset, the motherboard exposes an option to disable APM (Application Power Management). Most overclocking guides suggest disabling APM for better stability, at least initially. Among these is the official AMD FX Performance Tuning Guide, pages 5 and 10. Page 5 states:
Since APM sets a predefined TDP limit it is usually recommended to disable both AMD Turbo Core Technology and APM features when increasing the CPU frequency and voltage above the default levels.
Ron's Tech Tips also has this to say:
In a nutshell, AMD Application Power Management BIOS setting ensures the CPU stays within the 125W (8 core) or 95W (4 and 6 core) TDP the chip was designed for. I have seen many say that APM causes the CPU to throttle, this is both true and false. It is true that sometimes APM causes this, but throttling is not what it always does. there are times where it will slightly lower voltage while keeping the CPU at a higher clock rate.
All emphasis are mine.
Additionally, most enthusiast motherboards nowadays also offer a feature called Load-Line Calibration (LLC). According to a user posting in Linus Tech Tips' forums:
Vdroop is a drop in voltage supplied to the CPU as load increases; basically when you go from idle to load, the voltage would decrease. Given the small voltage tolerance that overclockers are working with (increased voltage is proportional to the CPU frequency/multiplier that an overclock can achieve), a droop in voltage applied to CPU can make a theoretically stable overclock unstable (dropping the voltage below that required to achieve the set frequency)
The following is the difference between defined (X) and measured (Y) vcore values without LLC:
Notice how the actual vcore value is always below what we expect.
In the next image, we can see that for that specific CPU (i7 3930K) and MoBo (Asus Rampage IV Extreme), an LLC setting of "High" (meaning a value of 50%) is enough to compensate for vdroop:
What I'm wondering is whether it is preferable to disable APM, and (most probably) settle with a lower LLC level (sometimes it's not needed at all), or to keep APM enabled and having to resort to a higher LLC setting to keep everything stable. My concerns are, in that order:
- System stability
- Calculation integrity
- System durability (less important)
- Thermal output and power consumption (even less important)
The reason for asking this is because a higher LLC setting introduces short voltage spikes to the CPU core, as already noted in this question: < Is it better to use an offset or manually set CPU voltage (with respect to CPU longevitity)? >, as well as in this Master's Lair post. Quoting:
If you have a decent motherboard, load-line calibration really doesn't buy you anything in terms of a higher overclock (...). It only artificially lowers the vcore that you'll have to set in your BIOS, but the CPU will still require the same amount of voltage when it's put under a load.
I'd recommend leaving [LLC] disabled unless you think that you're having a hard time achieving the overclock that you wanted and suspect excessive vdroop to be the problem.
On the one hand, I suspect APM does more than "just" enforcing an overall TDP ceiling and thus should rather be kept enabled if possible, despite suggestions to the contrary. But on the other, it also appears APM would introduce instability and thus require a higher LLC setting, which would itself be probably worse.
- CPU: FX-6350 @ 4.8 GHz (default is 3.9) - Motherboard: Asus Sabertooth 990FX R2.0 - Turbo Core: Off - CPU Offset Voltage: +0.09375v - APM Master Mode: On - C1E, C6 State, Cool'n'Quiet: All enabled (On) - CPU Load Line Calibration: Ultra High (75%) - CPU Power Phase Control: Standard - CPU Power Duty Control: C.Probe (Current) - Spread Spectrum is Off for CPU, CPU-NB and VRM.
I was previously running with LLC High (50%) for this clock speed, but got calculation error in Prime95 after 4h30min of testing, even with 100mv (+0.1) vcore offset.
Then I lowered the offset by 6.25mv and changed LLC to Ultra High, errors are gone.
This however pushed the load voltage up by 20mv in average - and 12mv spike (resulting in 1.488v) during certain load transitions, which is a little higher than ideal.
CPU Temperature was 63ºC maximum, after many hours of number crunching during the day. This is an air-cooled system (respectable cooler though, the Hyper 212X), worked well for 2 years with a -85mv offset (undervolt)
I'd like it to keep working for at least one more year.