Odd CPU Behaviour When Using Dedicated GPU

[cnelson]

Hey everyone, new guy here.

 

So I've been having this weird problem with my laptop for about a year. The CPU sometimes -seemingly at random- cuts power consumption way down when using dedicated GPU (Nvidia GT 635M) on games. By sometimes I actually mean entire sessions, not at random times. It will be fine for weeks, especially because I never shut down the laptop once it starts working, and then one day this issue will comeback again after a restart, persisting whenever I launch a game using the Nvidia GPU.

 

It originally showed itself through huge fps drops in games. The pattern didn't fit CPU throttling and it usually seemed to come back after some updates to a particular game which made me think for a while that the game or the gpu driver was the issue. Then I realised it happened in all games, from time to time.

 

Finally I was able to find the real cause of the problem, though I still have no idea what causes it: When a game is launched using the dedicated GPU, CPU power consumption will drop to around 7-8 watts and clock speeds to around 1.2-1.5 GHz. This is compared to around 32-35 watts and 2.4 - 3.4 (with turbo) GHz when using Intel HD graphics.

 

I've used Throttlestop to see if it could help, but at full throttle (3.4 Ghz with turbo) it was only able to get it up to around 18-20 watts with no noticable difference in frame rates (around 13-15 fps). For comparison the Intel HD 4000 will run it at around 30-33 fps. It used to be around 44 fps but I guess I need a disk cleanup and/or disk fragmentation or something. When the Nvidia GPU works the fps for that particular game is around 55 fps (I've seen mid-70s after a system format).

 

So here I am, begging anyone with any knowledge and/or experience as to what might be causing it. The system is practically a potato, though it still does alright:

 

- i7 3630QM 2.4 Ghz (3.4 with turbo)

- 8gb 1066 DDR3 RAM

- Nvidia GT 635M GPU

- Win10 64bit

- No clue what the mobo is. Just some generic laptop one.

 

Please, please, please don't come up with suggestions regarding drivers, temperatures and other incredibly superficial advice. I've already ruled out those months ago.

 

Looking forward to some constructive discussions and tips,

Cheers.

[genexis_x]

Is the laptop plugged in? If yes, the charger may having some problem.

If no, either the mobo or the battery is having power delivery issues.

[cnelson]

22 minutes ago, ZM Fong said:

Is the laptop plugged in? If yes, the charger may having some problem.

If no, either the mobo or the battery is having power delivery issues.

Battery is disconnected, always plugged in.

 

Power delivery shortage is something I've thought about, but it doesn't explain the cyclical nature of the problem. If this was caused by a power shortage I would be having this issue all the time, not some of the time.

[unclewebb]

9 hours ago, cnelson said:

It originally showed itself through huge fps drops in games. The pattern didn't fit CPU throttling

Severe CPU throttling can cause huge FPS drops.  Can you post a screenshot of ThrottleStop so I can see how you have the program setup? 

 

Can you run a log file while you are gaming so you have a record of what is going on?  Before you start logging, go into the ThrottleStop Options window and click on the Nvidia GPU option so your log file includes GPU temperature information.  After you are finished testing, exit ThrottleStop so it can write the log file data to a file.  Copy and paste the log file info somewhere convenient like www.pastebin.com and then post a link here so I can have a look.

 

Some laptops are designed so they start throttling when the battery is removed so something like that might be triggering this slow down.  If you use ThrottleStop to try and force the CPU to its full rated speed, this might trigger throttling of the GPU.  The GPU temperature while playing should give some indication if this is going on.  How many watts is your power adapter rated at?  

 

The other thing to check is what C0% does ThrottleStop report when your laptop is idle with nothing extra running?  Windows 10 has a lot of random background tasks that can significantly reduce performance.  C0% should be under 1.0% at the desktop.  An average around 0.5% would be even better.

 

[cnelson]

9 hours ago, unclewebb said:

Some laptops are designed so they start throttling when the battery is removed so something like that might be triggering this slow down... How many watts is your power adapter rated at?  

That's not the case. I doubt that's even a thing. All the power setting in Windows are maxed for high performance. Are you sure you don't mean "when unplugged from power"? Which wouldn't be a problem anyway, because I'd just change the power settings to remove that effect, if I was running it on battery power alone.

 

As per the adapter wattage; it's 90W. The laptop requires 90W (at least that's what it says on the label inside the machine).

 

9 hours ago, unclewebb said:

The other thing to check is what C0% does ThrottleStop report when your laptop is idle with nothing extra running?  Windows 10 has a lot of random background tasks that can significantly reduce performance.  C0% should be under 1.0% at the desktop.  An average around 0.5% would be even better.

When the computer is idling on desktop with only Mozilla and GeForce Experience running (as well as other stuff in the background) the C0% is between 0.8-1.5%, with an average of 1%.

 

Now to the screenshots:

 

Game lobby while running GPU with Throttlestop off:

 

 

 

Game lobby on GPU, Throttlestop on:

 

 

 

In game, GPU, Throttlestop on (FPS counter on top left):

 

 

 

Game end (roughly 10 minutes), GPU, Throttlestop on:

 

 

 

For comparison, this is in game, on CPU, with Throttlestop off (FPS counter top left):

 

 

TRL:

 

 

 

TPL:

 

 

 

Here are the links for the logs. One with the GPU running the graphics and one with the CPU running it:

CPU

GPU

 

Let me know what you think.

[cnelson]

Hmm... After going through the logs myself, it is clear that there's a 20-25 degrees difference between CPU temperatures, on GPU vs off.

 

My previous experience with CPU throttling (in a previous laptop) was a cyclical wave of fps drops in games every 30 secs or so. CPU would be throttled every once in a while to keep the temps low, once they're low enough it'd go back to normal and start again when they get high.

 

This might be a case of CPU temps being so high that it's constantly throttled, not because the CPU itself is causing the heat mind you, but the GPU. The same copper tubing cools both the CPU and the GPU, so presumably when the GPU starts getting hot, the CPU gets its fair share too, because its downstream the GPU, before the cooling fins.

 

This would explain why it happens at some times and not the others as well. Because now that I think about it, it's actually the winter months when I am able to enjoy games without huge fps drops while using GPU. Once ambient temperatures start getting higher, I start having this issue.

 

I might try modifying the internals and fit a separate cooling system for the GPU. I don't ever use the DVD drive anyway, I should be able to fit a laptop cooling fan in there with the necessary piping.

Anothe, albeit more expensive option would be to use a good cooling pad. I've taken off the bottom plate to help with the cooling, if there is something that blows air directly into it I reckon I would be able to use the GPU again

[cnelson]

Okay, I don't think heat is the problem.

 

I've just run a test with the laptop's bottom lid off and in front of a large room fan at full speed. The temperatures were down around 30 degrees (underload), but performance was still the same.

 

Here's the log:

GPU w/ Fan

[cnelson]

No one has anything to add?

[cnelson]

And just like that, it starts working again.

Why is this happening though??

 

(Interestingly it started working after I ran FSX. Coincidence?)