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blog: why-cores
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site/blog/why-cores.md
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site/blog/why-cores.md
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---
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title: "Why more `/sys/class/hwmon/*/temp*_label` than CPU cores?"
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date: 2025-06-05
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keywords:
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- hardware
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---
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So, I am currently working on a Linux tool+daemon to replace
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[`auto-cpufreq`](https://github.com/AdnanHodzic/auto-cpufreq) with a more
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efficient, tunable and observable alternative.
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And obviously, to tune a CPU well, knowing how hot it is is a requirement.
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Then, how do you actually see how hot your CPU is in Linux? Using
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`/sys/class/hwmon` of course.
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This path contains hardware monitoring devices. In the laptop I'm testing this
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on, `hwmon4` under this directory corresponds to the CPU.
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Let's run a `tree`:
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```text
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/sys/class/hwmon/hwmon4 -> ../../devices/platform/coretemp.0/hwmon/hwmon4
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├── device -> ../../../coretemp.0
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│ ├── driver_override
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│ ├── hwmon
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│ ├── modalias
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│ ├── power
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│ ├── subsystem -> ../../../bus/platform
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│ └── uevent
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├── name
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├── power
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│ ├── autosuspend_delay_ms
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│ ├── control
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│ ├── runtime_active_time
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│ ├── runtime_status
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│ └── runtime_suspended_time
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├── subsystem -> ../../../../../class/hwmon [recursive, not followed]
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├── temp10_crit
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├── temp10_crit_alarm
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├── temp10_input
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├── temp10_label
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├── temp10_max
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├── temp14_crit
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├── temp14_crit_alarm
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├── temp14_input
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├── temp14_label
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├── temp14_max
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├── temp18_crit
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├── temp18_crit_alarm
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├── temp18_input
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├── temp18_label
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├── temp18_max
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├── temp1_crit
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├── temp1_crit_alarm
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├── temp1_input
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├── temp1_label
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├── temp1_max
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├── temp22_crit
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├── temp22_crit_alarm
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├── temp22_input
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├── temp22_label
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├── temp22_max
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├── temp26_crit
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├── temp26_crit_alarm
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├── temp26_input
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├── temp26_label
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├── temp26_max
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├── temp2_crit
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├── temp2_crit_alarm
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├── temp2_input
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├── temp2_label
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├── temp2_max
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├── temp30_crit
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├── temp30_crit_alarm
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├── temp30_input
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├── temp30_label
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├── temp30_max
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├── temp34_crit
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├── temp34_crit_alarm
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├── temp34_input
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├── temp34_label
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├── temp34_max
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├── temp35_crit
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├── temp35_crit_alarm
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├── temp35_input
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├── temp35_label
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├── temp35_max
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├── temp36_crit
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├── temp36_crit_alarm
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├── temp36_input
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├── temp36_label
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├── temp36_max
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├── temp37_crit
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├── temp37_crit_alarm
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├── temp37_input
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├── temp37_label
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├── temp37_max
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├── temp38_crit
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├── temp38_crit_alarm
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├── temp38_input
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├── temp38_label
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├── temp38_max
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├── temp39_crit
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├── temp39_crit_alarm
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├── temp39_input
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├── temp39_label
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├── temp39_max
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├── temp40_crit
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├── temp40_crit_alarm
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├── temp40_input
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├── temp40_label
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├── temp40_max
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├── temp41_crit
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├── temp41_crit_alarm
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├── temp41_input
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├── temp41_label
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├── temp41_max
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├── temp42_crit
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├── temp42_crit_alarm
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├── temp42_input
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├── temp42_label
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├── temp42_max
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├── temp43_crit
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├── temp43_crit_alarm
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├── temp43_input
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├── temp43_label
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├── temp43_max
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├── temp44_crit
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├── temp44_crit_alarm
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├── temp44_input
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├── temp44_label
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├── temp44_max
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├── temp45_crit
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├── temp45_crit_alarm
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├── temp45_input
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├── temp45_label
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├── temp45_max
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├── temp46_crit
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├── temp46_crit_alarm
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├── temp46_input
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├── temp46_label
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├── temp46_max
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├── temp47_crit
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├── temp47_crit_alarm
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├── temp47_input
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├── temp47_label
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├── temp47_max
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├── temp48_crit
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├── temp48_crit_alarm
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├── temp48_input
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├── temp48_label
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├── temp48_max
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├── temp49_crit
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├── temp49_crit_alarm
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├── temp49_input
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├── temp49_label
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├── temp49_max
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├── temp6_crit
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├── temp6_crit_alarm
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├── temp6_input
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├── temp6_label
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├── temp6_max
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└── uevent
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```
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Let's `cat` all the `_label` files:
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```text
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/sys/class/hwmon/hwmon4/temp1_label:
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Package id 0
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/sys/class/hwmon/hwmon4/temp2_label:
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Core 0
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/sys/class/hwmon/hwmon4/temp6_label:
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Core 4
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/sys/class/hwmon/hwmon4/temp10_label:
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Core 8
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/sys/class/hwmon/hwmon4/temp14_label:
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Core 12
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/sys/class/hwmon/hwmon4/temp18_label:
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Core 16
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/sys/class/hwmon/hwmon4/temp22_label:
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Core 20
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/sys/class/hwmon/hwmon4/temp26_label:
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Core 24
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/sys/class/hwmon/hwmon4/temp30_label:
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Core 28
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/sys/class/hwmon/hwmon4/temp34_label:
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Core 32
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/sys/class/hwmon/hwmon4/temp35_label:
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Core 33
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/sys/class/hwmon/hwmon4/temp36_label:
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Core 34
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/sys/class/hwmon/hwmon4/temp37_label:
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Core 35
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/sys/class/hwmon/hwmon4/temp38_label:
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Core 36
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/sys/class/hwmon/hwmon4/temp39_label:
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Core 37
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/sys/class/hwmon/hwmon4/temp40_label:
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Core 38
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/sys/class/hwmon/hwmon4/temp41_label:
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Core 39
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/sys/class/hwmon/hwmon4/temp42_label:
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Core 40
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/sys/class/hwmon/hwmon4/temp43_label:
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Core 41
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/sys/class/hwmon/hwmon4/temp44_label:
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Core 42
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/sys/class/hwmon/hwmon4/temp45_label:
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Core 43
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/sys/class/hwmon/hwmon4/temp46_label:
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Core 44
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/sys/class/hwmon/hwmon4/temp47_label:
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Core 45
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/sys/class/hwmon/hwmon4/temp48_label:
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Core 46
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/sys/class/hwmon/hwmon4/temp49_label:
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Core 47
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```
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Notice something? I do:
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- `temp1_label` is `Package id 0` - what is that?
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- Other core numbers make no sense. This device only has 32 cores, there
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shouldn't be any gaps within numbers and the numbers shouldn't go that high.
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The explanation for the first point is simple, looking at the kernel
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[`coretemp.c`](https://github.com/torvalds/linux/blob/ec7714e4947909190ffb3041a03311a975350fe0/drivers/hwmon/coretemp.c#L348)
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implementation, we can see that it is the temperature of the CPU as a whole:
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```c
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static ssize_t show_label(
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struct device *dev,
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struct device_attribute *devattr,
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char *buf
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) {
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struct platform_data *pdata = dev_get_drvdata(dev);
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struct temp_data *tdata = container_of(devattr, struct temp_data, sd_attrs[ATTR_LABEL]);
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if (is_pkg_temp_data(tdata))
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return sprintf(buf, "Package id %u\n", pdata->pkg_id);
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return sprintf(buf, "Core %u\n", tdata->cpu_core_id);
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}
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```
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That leaves us the second question. Why do we have CPUs 1, 2, 6, 10, 14, 18, 22,
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26, 30, 34-49, instead of the expected 0-31?
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It turns out that CPU It's common for chip manufacturers to disable faulty or
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degraded cores before shipping. Or if a lower tier SKU[^Stock Keeping Unit] is
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selling more, cores of higher tier SKUs are disabled to match expectations.
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Most likely the 32 core CPU I was testing this on was actually just the 64 core
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version with a bunch of the cores disabled.
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