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Undervolting your CPU for fun and profit

Disclaimer

I'm not responsible if you ruin your system, this guide functions as documentation for future me. While this should just cause MCEs or system resets if done wrong it might also be able to ruin things in a more permanent way.

Why do this

It lowers temps generally and if you hit thermal throttling (as happens with my 5th Generation X1 Carbon) you may thermally throttle less often or at a higher frequency.

Prework

Repaste first, it's generally easier to do and can result in better gains (and it's all about them gains). For instance, my Skull Canyon NUC was resetting itself thermally when stress testing. Repasting lowered the max temps by 20°C.

Undervolting - The How

I based my info on https://github.com/mihic/linux-intel-undervolt which seems to work on my Intel Kaby Lake based laptop.

Using the MSR registers to write the values via msr-tools wrmsr binary.

The following python3 snippet is how I got the values to write.

# for a -110mv offset I run the following
format(0xFFE00000&( (round(-110*1.024)&0xFFF) <<21), '08x')

What you are actually writing is actually as follows, with the plane index being for cpu, gpu or cache for 0, 1 or 2 respectively.

constant	plane index	constant	write/read	offset
`80000`	`0`	`1`	`1`	`F1E00000`

So we end up with 0x80000011F1E00000 to write to MSR register 0x150.

Undervolting - The Integration

I made a script in /opt/bin/, where I place my custom system scripts called undervolt.sh (make sure it's executable).

#!/usr/bin/env sh
/usr/sbin/wrmsr 0x150 0x80000011F1E00000  # cpu core  -110
/usr/sbin/wrmsr 0x150 0x80000211F1E00000  # cpu cache -110
/usr/sbin/wrmsr 0x150 0x80000111F4800000  # gpu core  -90

I then made a custom systemd unit in /etc/systemd/system called undervolt.service with the following content.

[Unit]
Description=Undervolt Service

[Service]
ExecStart=/opt/bin/undervolt.sh

[Install]
WantedBy=multi-user.target

I then systemctl enable undervolt.service so I'll get my settings on boot.

The following script was also placed in /usr/lib/systemd/system-sleep/undervolt.sh to get the settings after recovering from sleep, as they are lost at that point.

#!/bin/sh
if [ "${1}" == "post" ]; then
  /opt/bin/undervolt.sh
fi

That's it, everything after this point is what I went through in testing.

Testing for stability

I stress tested with mprime in stress mode for the CPU and glxmark or gputest for the GPU. I only recorded the results for the CPU, as that's what I care about.

No changes:

15-17W package tdp
15W core tdp
3.06-3.22 Ghz

50mv Undervolt:

15-16W package tdp
14-15W core tdp
3.22-3.43 Ghz

70mv undervolt:

15-16W package tdp
14W core tdp
3.22-3.43 Ghz

90mv undervolt:

15W package tdp
13-14W core tdp
3.32-3.54 Ghz

100mv undervolt:

15W package tdp
13W core tdp
3.42-3.67 Ghz

110mv undervolt:

15W package tdp
13W core tdp
3.42-3.67 Ghz

started getting gfx artifacts, switched the gfx undervolt to 100 here

115mv undervolt:

15W package tdp
13W core tdp
3.48-3.72 Ghz

115mv undervolt with repaste:

15-16W package tdp
14-15W core tdp
3.63-3.81 Ghz

120mv undervolt with repaste:

15-16W package tdp
14-15W core tdp
3.63-3.81 Ghz

I decided on 110mv cpu and 90mv gpu undervolt for stability, with proper ventilation I get about 3.7-3.8 Ghz out of a max of 3.9 Ghz.

Other notes: Undervolting made the cpu max speed less spiky.

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