Thermal throttling is a technology found in the Intel Pentium 4 CPUs that slows down the instruction execution rate of the CPU when the internal temperature exceeds an internal limit. Thermal throttling can be triggered by excess temperature reported by internal sensors in the core or on the chip or by an external sensor typically mounted on the motherboard. These sensors are not directly coupled and will typically exhibit different temperature profiles under changing loads. When fully engaged, throttling will cut the execution rate and heat generated by about 50%. The slow down may be less because a CPU that is generating only a little more heat than it can handle will slip in and out of thermal throttling fairly rapidly.

Thermal throttling in the Pentium 4 does not slow the CPU clock rate. It turns the clock on and off every few microseconds. The effect is the same. Gates are switched less frequently and less heat is generated.

The onset of thermal throttling in the Pentium 4 is set at 70 degrees Centigrade and is not user controllable. The only user controllable parameter is whether the external sensor is used in throttling.

Thermal throttling will allow a CPU to continue running with some performance degradation in unusual situations such as those caused by an exceptionally heavy CPU loads or a very hot day.

Thermal throttling may not prevent the CPU from overheating severely if the normal heat removal is severely impaired -- e.g. the heatsink has come loose or the CPU fan has stopped. A shutdown mechanism is still required should the temperature get too high. This is said to occur at 135C

If other thermal controls such as variable fan speeds are present in the overall system, they generally need to be integrated with the CPU thermal monitoring. For example, a fan speed increase based on temperature probably should be set to kick in well below 70C If it is not, thermal throttling will set in before the fan speeds up.

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