4 ms·

Thermal sensors measure temperature in specific points / small areas. There are a finite number of these and they do not cover the entire CPU / GPU / motherboar

Thermal sensors measure temperature in specific points / small areas. There are a finite number of these and they do not cover the entire CPU / GPU / motherboard / etc. If one puts a massive load on a CPU, the temperature spikes quickly and widely across enough of the die that the sensors work as expected and the system throttles / shuts down. If one puts a small load on the CPU for a long period of time, areas that are not covered by sensors may overheat, damaging the CPU. This is true for other components in the system, too. If the laptop is out in the air, convection will remove enough heat to prevent these invisible "hot spots". The bag acts as a blanket. Over hours, the laptop cooks parts of itself to death[0] in ways the system cannot detect. [0] Or damages them in ways that make them unstable. Edited to add [0]

Buttons840 5y ago
Do they not put thermal sensors in the CPU? My laptop gives a temperature for each individual CPU core. Whenever I start a heavy CPU task I instantly see a temperature spike. The theory that the thermometers are in potentially useless spots doesn't match my observations.
- daggersandscars 5y ago
  My apologies for not being clearer that this is not only about the CPU. It's about all the components in the laptop, not just the CPU. I used the CPU in the example because it's a component most of us are familiar with. It's not that the sensors are in useless spots. They're the best sensors in the best spots the designers could make work for the price point they're trying to hit. To answer your CPU question: CPUs have thermal sensors, probably the highest number in the machine as they work with clock speed scaling and the CPU is in the top 2 most expensive components. But each of these sensors is covering a large area, which may not heat up evenly under low loads. A heavy load is the easy case and the one they're designed to detect: the CPU core is fully loaded, it's generating maximum heat, and the entire core area will heat up quickly. A light load is harder to detect. Depending on die design and sensor placement, one corner of a core may heat up in a way the sensor doesn't detect well at low CPU loads. These sensors are also only accurate "enough". This corner of a core may have to get to 110C before the sensor realizes it's overheating and throttles it. It could sit for hours at 105C, very slowly toasting itself. But CPUs are, generally, actively cooled or have large heatsinks, so this is less of an issue in practice... though wrapping a laptop in a blanket overnight might make it an issue. To move on beyond CPUs: There are many thermally sensitive components in a laptop. Some of them have thermal sensors. Others do not. Almost all of them rely on convection to shed heat. Leaving the laptop effectively "on" and in a bag overnight may bring them to a temperature that damages them.
  - HPsquared 5y ago
    Light load over a long period of time will result in a fairly uniform temperature distribution. It sounds like the system must have no "general" temperature sensor measuring the air temperature inside the machine (or chassis temperature). Probably some small component on the logic board generates a bit of heat, requiring a certain "internal ambient" temperature, and the designers (never having thought to test it with restricted airflow) never noticed the implicit assumptions they had made.
sbakzbsmx 5y ago
A single sensor by the exhaust fan would be an excellent proxy for the T anywhere in the laptop. The really hot stuff have sensors themselves, so if anything else is hot it will be revealed in the exhaust
- stagger87 5y ago
  An exhaust fan doesn't work in a bag.
  - Tagbert 5y ago
    That is why the temp sensor would quickly register high heat and shut down the system.
    - deleted 5y ago
      [deleted]
jerf 5y ago
Thank you. Good point. I still wonder if machines should be designed to deal with this, and would still be tempted to call it a "warranty issue" that they didn't handle a scenario they're clearly aware of. On the other hand it would also be very nice if software just didn't decide it can unsuspend a laptop whenever, and then apparently do hours of work without ever re-suspending, so... I guess you could say I've got enough blame to ladle around for everybody.

Buttons840 5y ago

Do they not put thermal sensors in the CPU? My laptop gives a temperature for each individual CPU core. Whenever I start a heavy CPU task I instantly see a temperature spike. The theory that the thermometers are in potentially useless spots doesn't match my observations.

daggersandscars 5y ago
My apologies for not being clearer that this is not only about the CPU. It's about all the components in the laptop, not just the CPU. I used the CPU in the example because it's a component most of us are familiar with. It's not that the sensors are in useless spots. They're the best sensors in the best spots the designers could make work for the price point they're trying to hit. To answer your CPU question: CPUs have thermal sensors, probably the highest number in the machine as they work with clock speed scaling and the CPU is in the top 2 most expensive components. But each of these sensors is covering a large area, which may not heat up evenly under low loads. A heavy load is the easy case and the one they're designed to detect: the CPU core is fully loaded, it's generating maximum heat, and the entire core area will heat up quickly. A light load is harder to detect. Depending on die design and sensor placement, one corner of a core may heat up in a way the sensor doesn't detect well at low CPU loads. These sensors are also only accurate "enough". This corner of a core may have to get to 110C before the sensor realizes it's overheating and throttles it. It could sit for hours at 105C, very slowly toasting itself. But CPUs are, generally, actively cooled or have large heatsinks, so this is less of an issue in practice... though wrapping a laptop in a blanket overnight might make it an issue. To move on beyond CPUs: There are many thermally sensitive components in a laptop. Some of them have thermal sensors. Others do not. Almost all of them rely on convection to shed heat. Leaving the laptop effectively "on" and in a bag overnight may bring them to a temperature that damages them.
- HPsquared 5y ago
  Light load over a long period of time will result in a fairly uniform temperature distribution. It sounds like the system must have no "general" temperature sensor measuring the air temperature inside the machine (or chassis temperature). Probably some small component on the logic board generates a bit of heat, requiring a certain "internal ambient" temperature, and the designers (never having thought to test it with restricted airflow) never noticed the implicit assumptions they had made.

sbakzbsmx 5y ago

A single sensor by the exhaust fan would be an excellent proxy for the T anywhere in the laptop. The really hot stuff have sensors themselves, so if anything else is hot it will be revealed in the exhaust

stagger87 5y ago
An exhaust fan doesn't work in a bag.
- Tagbert 5y ago
  That is why the temp sensor would quickly register high heat and shut down the system.
  - deleted 5y ago
    [deleted]

jerf 5y ago

Thank you. Good point. I still wonder if machines should be designed to deal with this, and would still be tempted to call it a "warranty issue" that they didn't handle a scenario they're clearly aware of. On the other hand it would also be very nice if software just didn't decide it can unsuspend a laptop whenever, and then apparently do hours of work without ever re-suspending, so... I guess you could say I've got enough blame to ladle around for everybody.