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Topic Title: Temperature Sensors Differences
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Created On: 01/10/2009 10:08 AM
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 01/10/2009 10:08 AM
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varao
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Joined: 01/10/2009

I'm trying to find out how the differences in temperature sensor readings can be explained.

I'm running Linux on an HP 6715b (TL-60).

With lm_sensors, I'm getting access to sensors through 2 different access paths, acpi and libsensors.

ACPI provides a (general) CPU temperature reading. That's all.

libsensors provides:

- core0
- core0
- core1
- core1
- m/b
- cpu

I'd like to know why each core has 2 sensors (different locations?). If I look at the readout from libsensors, I get:

- core0 42 C
- core0 38 C
- core1 42 C
- core1 35 C
- m/b 45 C
- cpu 43 C

Now the CPU reading from acpi is 40 C (while the readout from libsensors is 43 C).

What explains the differences between these temperature readings? For instance, why is core0 42 C from one readout while it's 38 C from another readout? Where are these sensors located?

Thanks.
 01/11/2009 06:28 AM
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chiplist
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Joined: 12/29/2006

They're caused by measuring on different locations but also by variations within the measuring devices. As long as these values are not too far apart, the differences are nothing to worry about. I don't think that manufacturers even specify temperature ranges/differences per core in their documentation.

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Adrian Offerman
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 01/13/2009 01:39 PM
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444
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It is not unusual for these sensors to be mislabeled under Linux. There is one sensor per core (each can have different readings), and any number of sensors on the motherboard (one in the cpu socket, maybe another near the chipset, etc). The core sensors can be identified by that fact that they fluctuate very quickly; putting the system under load will instantly cause a 10-20c rise, whereas the external cpu sensor on the motherboard would take a minute or two to rise.
 01/22/2009 07:41 AM
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varao
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The impression left by these readings is that there's 2 sensors per core. The first sensor is always higher than the 2nd sensor. These values differ by as much as 7 degrees. I thought I could find out where the sensors are located.

I did notice the other day after booting the system that my cores were easily above 80 C. At that point I noticed my fan wasn't running. I wasn't doing anything with the system and the temperature slowly rose to above 100 C. I took a screenshot and then proceeded to shutdown. Halfway into shutdown and the system itself generated a message "Critical temperature reached (88 C) ..." and halted the system.

(my fan is working in the mean time, must have been a cosmic ray).

The interesting thing here is that the 1st reading (CPU) never rose above 43 C. Even though all core's were providing values above 100 C, the CPU reading never went over 43 C. When the fan is running, the CPU reading can easily reach 60 C. I assume that the CPU reading is 'excited' when the fan blows, i.e. the sensor is located along the path of the airflow.

The fan not running must have been some coincidence/quirk. I realise the CPUs are rated to 95 C. Begs the question which sensor triggers the shutdown/shutoff?

Thanks for the input.
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