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46 lines
2.1 KiB
Markdown
46 lines
2.1 KiB
Markdown
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---
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title: "Disabling CPUs to Save Power"
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date: 2024-04-06T20:48:52-04:00
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draft: false
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tags: []
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math: false
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medium_enabled: false
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---
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Looking for ways to reduce the power usage of my home server? This post shows a not-so-scientific look at how disabling CPUs can help save some power.
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I run a Dell PowerEdge R430 with an Intel Xeon E5-2643 v3 CPU. This gives me 12 physical cores, and with hyperthreading this presents itself as 24 logical cores.
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Given that I mostly use my server as media storage, most of those CPUs sit idle most of the time. My thought is, how much power can I save if I disable some of these unused cores?
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For idle workloads disabling the CPU does not result in any noticeable power savings. The power savings is significant, however, if you analyze the system under load.
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To conduct this experiment, I used a [Kill a Watt](https://en.wikipedia.org/wiki/Kill_A_Watt) measuring device which monitors the power usage of whatever is plugged into it.
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To disable a CPU in Linux, use the following command:
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```bash
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# Disable CPU 23
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echo 0 | sudo tee /sys/devices/system/cpu/cpu23/online > /dev/null
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```
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Repeat for any CPUs you want to disable. You can see which CPUs are available using `ls /sys/devices/system/cpu`. The `htop` tool will display in addition to the CPU utilization, which CPUs are offline.
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To get the system under load, I used the `stress` tool.
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```bash
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# Spin off 24 dummy tasks that max out each CPU
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stress -c 24
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```
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I ran the stress tool with the corresponding number of logical CPUs I had online to come up with the following table:
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| # Online Logical CPU | Power (Watts) |
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| -------------------- | ------------- |
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| 24 | 360 |
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| 12 | 295 |
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| 6 | 194 |
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| 4 | 173 |
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Cutting down to 4 logical CPUs can cut the power usage under load in half! Do note though, that this exchanges performance for power savings. If you are running a low amount of services on your home server or can wait a bit of extra time for a computation to finish, consider disabling some of your CPUs.
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