Which limit wins when several apply?
Power, current, temperature and the operating system each allow a top speed. The lowest one wins, and that limit is in control.
1. Each limit sets a top speed
Power, current, temperature (temp) and the operating system (OS) each allow a top speed, in GHz, like speed signs on a road. Unlike signs, these change many times a second.
2. The lowest one wins
A tiny computer in the chip, the power controller, picks the lowest speed, as a car driver obeys the lowest speed sign. So usually one limit is in control.
3. Reports name the limit in control
The chip's own reports (telemetry) usually show which limit is in control at each moment. Monitoring tools show this as the limit reason.
4. Low speed score? Ask which limit
When a speed test (a benchmark) gives a low score, find which limit was in control and ask whether it should be. If not, find out why: a wrong setting, poor cooling or a bug.
Words to know
- GHz
- Billions of clock ticks per second, a measure of chip speed.
- OS
- Operating system, such as Windows or Linux. It can ask for a lower speed to save battery.
- Current
- How much electricity flows through a wire, measured in amps.
- Telemetry
- Numbers a chip reports about itself, such as speed, power and temperature.
- Limit reason
- The name of the limit that is holding the speed down.
- Benchmark
- A standard test program that gives a speed score.
Where you meet it at work
When a benchmark score drops, a power engineer first asks which limit was in control. The recorded telemetry usually answers it.
Check yourself
Power allows 4.2 GHz, temperature allows 3.6 GHz, and the OS asks for 4.0 GHz. How fast does the chip run?Think first, then tap to see the answer.
3.6 GHz, the lowest. Temperature is in control.