Quad Core in T420 - Disable Cores to stay within TDP limits?

T400/410/420 and T500/510/520 series specific matters only
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andrewferguson
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Quad Core in T420 - Disable Cores to stay within TDP limits?

#1 Post by andrewferguson » Sun Aug 27, 2017 6:20 am

For a ThinkPad T420 with a quad-core CPU, where the TDP of the CPU (45W) is greater than the maximum supported TDP of the T420 (35W), would disabling two of the cores of the CPU (through this method) bring the CPU back within the 35W maximum? (A pointless question I know, as if you're disabling half of a quad core, you may as well use an dual core, but I'm curious).

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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#2 Post by Thinkpad4by3 » Sun Aug 27, 2017 8:13 am

Could you do it? Yes
Is it hard? Yes
Also, you can use TPFanControl to make the fan run harder for the hotter CPU.

Should you do it? No. A quad core run as a dual core is going to be less efficient than a dual core and it will still run hot.
Last edited by Thinkpad4by3 on Sun Aug 27, 2017 8:13 am, edited 1 time in total.
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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#3 Post by Cigarguy » Sun Aug 27, 2017 8:15 am

Unlike a desktop overclocking setup, it will be difficult to disable cores with a locked BIOS. Not much one can do about the cooling either. You might be able to do it via software. But, you're right, it's kind of a pointless exercise.

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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#4 Post by andrewferguson » Sun Aug 27, 2017 10:11 am

I've just realised that the link in my original post didn't work.
https://www.cyberciti.biz/faq/debian-rh ... -commands/
That's the website I meant to link to. It describes a method of disabling CPU cores (on Linux - which I use) that appears to work on other laptops I've tried it on, so in terms of difficulty of disabling cores it isn't difficult, but I don't know how effective it is.

Aside from disabling cores, are there any other methods for reducing the TDP of a CPU? The ideal scenario would be having the CPU act like a 35W dual core when the laptop is being carried around, but then having the ability to return to the quad i7 when the laptop is docked and more adequate cooling can be provided.

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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#5 Post by dr_st » Sun Aug 27, 2017 10:43 am

andrewferguson wrote:
Sun Aug 27, 2017 10:11 am
The ideal scenario would be having the CPU act like a 35W dual core when the laptop is being carried around, but then having the ability to return to the quad i7 when the laptop is docked and more adequate cooling can be provided.
Actually, most Thinkpads I know of run hotter when docked. I'd say cooling is less adequate.
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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#6 Post by axur-delmeria » Sun Aug 27, 2017 11:10 am

andrewferguson wrote:
Sun Aug 27, 2017 10:11 am
It describes a method of disabling CPU cores (on Linux - which I use) that appears to work on other laptops I've tried it on, so in terms of difficulty of disabling cores it isn't difficult, but I don't know how effective it is.
Run psensor (or whatever temperature monitoring software you have), then run 7zip in benchmark mode on a terminal:

Code: Select all

7z b n
where n is how many times the benchmark will repeat. 5-10 runs should be enough.
Do this before and after you disable the cores, so you have a baseline for comparison. Let the system temperatures return to normal (at idle) before running the 7z benchmark again.
Aside from disabling cores, are there any other methods for reducing the TDP of a CPU?
Disabling Turbo Boost? TLP can be set up to do so.
Another is to reduce the maximum clock speed. :O
The ideal scenario would be having the CPU act like a 35W dual core when the laptop is being carried around, but then having the ability to return to the quad i7 when the laptop is docked and more adequate cooling can be provided.
I don't have a solution, but since you're using Linux, one approach that could work is a script that checks whether the laptop is on battery or AC power, then disables/enables the cores as needed.
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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#7 Post by Sweater Fish Deluxe » Sun Aug 27, 2017 12:24 pm

andrewferguson wrote:
Sun Aug 27, 2017 10:11 am
Aside from disabling cores, are there any other methods for reducing the TDP of a CPU? The ideal scenario would be having the CPU act like a 35W dual core when the laptop is being carried around, but then having the ability to return to the quad i7 when the laptop is docked and more adequate cooling can be provided.
If that's your goal, the easiest way by far is just to throttle the clockspeed. Disabling hyperthreading also makes core i CPUs run a lot cooler, but I'm not sure there's a way of doing that outside of the BIOS, so you'd have to restart.

I agree with dr_st, though, Thinkpads generally seem to run hotter in a dock than undocked. This might be because I'm running internal as well as external monitors when docked, though. Maybe with a single monitor on a dock it wouldn't be an issue.

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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#8 Post by bit_twiddler » Sun Aug 27, 2017 1:27 pm

The TDP of a T420 might depend upon whether it has integrated graphics, or Nvidia.
I believe that they share a heatsink in the T420, so going with the HD3000 might
be a compensating factor.
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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#9 Post by andrewferguson » Sun Aug 27, 2017 4:13 pm

Thanks very much for the replies. I've tested disabling one core of a dual-core i5 with and then benchmarking with 7z (not on a TP, on a Dell), and it lowers the maximum temperature from 84 degrees C to just less than 70 degrees. From that, I think that disabling cores is a viable - if inefficient - way of reducing the TDP of a processor. Due to HyperThreading 4 CPUs appear, and they can be individually disabled. Disabling the HyperThreaded cores does make some difference (around 5 degrees) to the temperature, but disabling the core entirely is more effective.

"Dock" was a bad choice of wording, I meant when on a desk, with a better cooling setup, not a ThinkPad dock.

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Re: Quad Core in T420 - Disable Cores to stay within TDP limits?

#10 Post by jaspen-meyer » Sun Aug 27, 2017 5:33 pm

Thinkpad4by3 wrote:
Sun Aug 27, 2017 8:13 am
Could you do it? Yes
Is it hard? Yes
It's one boot line parameter.

You can limit cpu frequency, per core, here:
/sys/devices/system/cpu/cpu0/cpufreq
/sys/devices/system/cpu/cpu1/cpufreq
etc

For example:

Code: Select all

:/sys/devices/system/cpu/cpu0/cpufreq$ cat scaling_available_frequencies 
2267000 2266000 1600000 
:/sys/devices/system/cpu/cpu0/cpufreq$ echo 1600000 | sudo tee scaling_max_freq 
There is a one-pipe, a two-pipe, and a three-pipe heat sink for the T420. Be sure to use a 2-pipe model if you're using a quad core.
T420 i7 3612QM; T420 i7 3630QM; T400 Q9100; x60s libreboot, led; x24 xiphmont led

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