i5-8250U power limit throttling on AC power no matter what I do

Hello everybody,

I have a Xiaomi Mi Notebook Pro i5 edition with specs as in my signature. I usually run the laptop with the battery disconnected (to increase longevity), and unfortunately the i5-8250U throttles down to 15W.


I have set the Power Limits to about 44W, which is as much power as the 8250U will draw under a Prime95 Stress Test at its full 3.4GHz turbo.

I can confirm that an the battery the CPU will draw 40W alone constantly under a Prime 95 benchmark with the temperature peaking in the low 90s.

The CPU is definitely not thermal throttling. If I add a dGPU load (MX150), the CPU will power limit throttle down to 15W, even if I have set Power Limit 1 and 2 at 40W.

With the battery unplugged, the CPU will never go above 15W, not even for a millisecond.

So obviously there is a second value somewhere in the laptop that limits the power of the CPU.

I know the risks and why Xiaomi decided to limit power like this, because the 65W included USB-C power adapter is underpowered for the laptop. When I run off AC power with no battery and run an extremely demanding game (Cities skylines pushes the CPU right up to 15W, GPU power limit throttles and pushes the GPU VRAM controller to 100%) the power brick becomes hot. Then after a period of time (sometimes 2 minutes, sometimes 20 minutes) The laptop abruptly powers off like it has lost power. I assume the 65W USB C adapter is not enough to power the laptop fully and overcurrent limit protection in the adapter stops the power. If I plug in battery and run Cities Skylines I can play it for over an hour, so the motherboard isn't being overloaded.

Does anybody know any alternate registers where the TDP is set? TPL in ThrottleStop doesn't do anything, and neither does Intel XTU.

 

IIRC, Xiaomi released a bios update which stops you from changing the power limits. Techtablets on youtube has a video on this.

 

Dear God......

 

Not unreasonable, to be fair to Xiaomi. The MX150 draws around 30W, the i5 8250u will draw 44W, everything else will draw about 5W (DRAM, ssd, screen etc), resulting in a 79W total power draw, which is much greater than the 65W power supply. The real stupidity was not designing a charging circuit that could deliver 90W and increasing the power limits from the factory, or at least not locking the power limits. It's a shame because the cooling on this laptop is actually really good-how many other laptops would be able to handle such a TDP increase?

 

Lol. I actually initially had the BIOS update that locked undervolting. I downgraded it.

I know.

The cooling is unbelieveable for a 1000 AUD laptop. I can confirm the MX150 is power limit throttling. I don't mind that, since I don't have any games that require extreme GPU power. Plus I can overclock +1000MHz on the VRAM and 250MHz on the core. I was thinking of buying an Apple USB-C 87W charger. Shame about the cost.

Update: The CPU will throttle down when the GPU is fully loaded. It fluctuates between 35W and 15W with PL1 and VR Current Throttling flashing erratically. It means there is some othetr vaule for PL1 I'm not aware of, since my PL1 in RW-Everything is already set sky high to 44W. I'm getting VR Current Throttling in ThrottleStop and HWInfo, and I think that's the absolute limit of the laptop. If the voltage regulators are working at their max value, I think I've hit the hard limit for the electrical design of my laptop, and no further optimization will help without damaging the circuitry. Keep in mind that at max power, the laptop is drawing (presumably) the maximum of 65W from the charger+ XX watts from the battery.

Obviously I'm asking a bit much for my laptop to have a CPU drawing 44W + 30W GPU fully maxed out and not throttling. My friend has a 2500AUD Dell XPS whose 8550U throttles right down to 800MHz within 3 seconds of running Prime95 with its Voltage Regulators thermal throttling. My $1000 laptop sustains 40+W on the CPU with absolutely no throttling whatsoever (when the GPU isn't loaded)

I'm only asking this because I think there is another PL1 limit set somewhere. The 8250U, with the battery plugged in, is capable of drawing 20W for tiny bursts (~0.2s) when opening Edge, for example, but quickly drops down to sub-15W levels. But with the battery not plugged in, the CPU will not, for one nanosecond, go a single watt above 15W. With Speedshift, having tiny bursts of high power is more efficient and the difference is noticeably in daily tasks.

 

Many laptops have a hard coded PL1 limit set in the BIOS for when the laptop is on battery. My laptop's method of throttling when on battery is slightly different-my reseller disabled speedshift and capped the clockspeed to 1.6GHz for all 4 cores. When I was plugged in there was absolutely no throttling. I got around this by using throttlestop to reenable speedshift and now my laptop is super responsive.

 

PL1 limit on battery is controlled by the EC. Ain't no way around that without a Prema Bios. Even if you disable "PECI" in your Bios, that won't solve anything, because that PECI setting is only for dealing with temps and fan controls. PECI power limits are controlled by the EC and are unaffected by the Bios setting.

 

You can actually control some parts of the EC. That's how NoteBook Fan Control controls laptop fans. If you can find a datasheet for the specific EC in your laptop (Highly improbable) and find that these registers are unlocked (Even more unlikely) then you might be able to change these values (just about impossible).


I'm seeing the exact opposite. My CPU can sustain 40W on battery power alone, but locks down to 15W without the battery.

 

You can't access PECI power limits that way. You're probably referring to MMIO power limits (which you can access through the "memory" tab in RW Everything).
I've never seen the EC ever have the PECI power limits accessible through EC RAM itself