Nvidia 840M throttle in AC mode but not battery mode

For the last few months I've been encountering a unusual issue with my 840m: when I'm not plugging in my AC charger, everythings works fine, but when I plug it in the gpu starts to throttle a lot, even when the temperature is a little far from temperature limit
I do notice that the higher the temperature the more likely it starts to throttle (around 75 degree), but since it doesnt happen in battery mode I dont know how to solve this
I've already set power mode to high performance in both windows and nvidia control panel, but no luck
Also this happen with all windows versions (8.1 and 10), all drivers

This is heaven running on battery


And this is heaven running on AC power


Yes, I did overclocked it but only to make the issue more clearly seen, it happens on default clocks as well as when underclocked, and sometimes doesnt on high temp at all

In GPU-Z PerfCaps Reason was: Pwr, which I believe is PowerLimits problem
How can I fix this and what's causing this problem?

 

Perfcap power means it's hitting the power limit of the vbios and so performance is being dialed back to stay within its rated TDP.

 

thanks, so why is this happenning when Im using AC line power? And can I mod the vbios to change the power limit?

 

When you're on AC power, the battery saver mode gets shut off. It doesn't look to me like you're taking a performance hit between either mode unless I'm missing something in the screenshots.

 

In the second screenshot, if you take a look at the GPU1 core clock, GPU1 ultilization limit and frametime and compare to the first screenshot, you will see the performance hit I took
It's like the 840m is being disabled and re-enabled almost at the same time, resulting in a shuttering image and fps spikes is everywhere
pullout the charger and everything come back to normal again
also when I stresstest the system with aida64, the gpu does thermal throttle but in a better way, as it just disable the boost clock

 

Use DDU to change drivers to 365.19 and see if it still does it. The 368 branch is a disaster area. You might even be better with the 362.00 driver.

I don't use Afterburner so I didn't take a good look at the stats. My guess is it's the drivers.

 

I'm downloading 362.00 driver now, but honestly the issue would still be there
I tried to install windows 8.1, and also the driver that came with the driver disc (around 328.xx driver I believe) and it throttle anyway
Edit: this is with 362.00 driver, I'm still getting Pwr perfcap and temperature is no where near high

 

Show me the same graph on battery please. Power limit means the card is throttling for hitting its TDP. Overclocks without an unlocked TDP will generally hit the power limit. You're pushing 1.13v through a low end Maxwell chip which will definitely cause it to power limit throttle. My 980Ms run their stock clocks including their boost clocks at 0.975v and at stock they run at 1.04 and 1.06v respectively. The only way to get the power limit throttle to go away is to reduce the power consumption. Voltage doesn't scale evenly like clocks do, you hit a point where you need dramatically higher voltage to hit a given clock speed. Try pulling back your overvolt or change the demand being put on the GPU by turning down GPU intensive calculations like anti aliasing and tessellation. There's unfortunately no unlocked vbios for that chip. With a 33W TDP, there's just not enough wiggle room to push it.

 

this is taken with the stock clock on my 840M, I stopped using OC since I found out about the throttling issue
I OCed about one and a half year with the gpu always drawing 1.13v and stay stable, until a few months back when this happened
Also I cant do anything about voltage on my notebook, both cpu and gpu
This screenshot is taken with ACS running on battery power, same voltage draw, same clock speed, and even doing a underclock still result in a 1.13v draw from the gpu


I guess that "change the demand being put on the GPU" is the only way to do it now