Does On-Demand Clock Modulation (ODCM) conserve battery?

Many low price laptop these days come with Intel Celeron M. Unlike its competitor, the AMD Mobile Sempron, Celeron M lacks dynamic frequency and voltage management. On processors like Mobile Sempron, Pentium M, Core Duo, and Core 2 Duo, the cpu drops frequency and voltage when there is less work load. So if nothing is going on, your cpu may drop from 1.8 Ghz to just 600 mhz and voltage may drop from 1.2v to 1.0v.

Celeron M in contrast runs at the same frequency and voltage all the time. This is why Celeron M typically have a shorter battery life than their mainstream cousins. Celeron M does have On-Demand Clock Modulation (ODCM). Can this be use to increase battery life?

How does ODCM work?

ODCM conjured up an image of a cpu dynamically changing the clock of the cpu depending on load. In fact, all ODCM does is force a processor to go into idle. Suppose you set the ODCM throttling to 75%. Your processor will work normally 75% of the time, and spend the other 25% slacking off. As a result, your cpu runs at the same clock speed and voltage, but runs slower because the cpu is forced to idle.

Does this actually save you power? Well, that depends on your usage. Suppose your cpu is running always at 100%, you are going to use a lot of power. Suppose you then set ODCM to throttle at 50%. 50% of the cycle will be idle, so you will use less power because the computer is at rest 50% of the time. However, the task will now take twice as long, this is not a good trade off.

If on the other hand, you’re going to do is type something in the computer, then your computer is probably idling most of the time waiting for you to press a key. In that case, power usage with ODCM and with ODCM is probably not all that different in both cases the computer is in idle most of the time. However, what if the forced idle uses less power than the natural idle state. Basically if you were to sit around and your computer goes into idle, does it use more power than if ODCM force the computer to idle? I decided to test this out.

The Test

The test platform consists of a Acer Aspire 3680 with a Celeron M 520 1.6 Ghz, 512 Mb RAM, and standard battery. I will run two trials.

1. Normal Test – Laptop is turned on and left alone until the computer powers off.
2. ODCM Test – Run RMClock and set it to throttle at 12.5%. Leave computer alone until the computer powers off. This causes the computer to go into force idle 87.5% of the time.

In both cases, we set the power manager to not turn off the monitor or hard disk. We also make sure nothing is schedule to run during the test. Wireless is turned off.

In both case, the computer powered off at about 2 hours. This mean that the forced idle uses the same amount of power as the normal idle. There was no power savings at all.

Conclusion

ODCM is not a useful tool for battery conservation. Most mobile processor conserve power by idling at a lower clock speed and voltage. ODCM is a poor substitute that attempts to force a processors to go into idle mode. However, since the force idle mode uses exactly the same power as the normal idle mode, you will not notice any power savings.