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Linux Meets Low Power

By John Blyler
LPD: How does Linux 6 work with low power. Do you turn off certain parts of the chip that aren’t being used?
Ready: There’s an analogy to the boot. It’s very dependent upon the capability of the underlying hardware and the mechanisms that are available to you. There is dynamic power management-that is, there is there is a specific interface in Linux called DPM. There is an abstract layer of power management that gets realized by the underlying hardware. There is a generic interface, too, but then you have to make it work for the specific hardware.

We helped develop that with IBM and others. What we ran into, however, was that the implementations of power management as seen in the hardware … the semiconductor makers would typically write their own set of drivers and or software, not necessarily DPM compliant, to make available power management capability. So you didn’t get a uniformity. That meant how you might do something on ARM was different than how you do it on PowerPC. We sort of gave up. You have to undo everything folks had already done and do it the generic way. It was too expensive.

Still, like the boot thing, we do inventory what is there, what mechanisms are available for individual hardware devices. And in the case of ARM, you can have a power utilization profile for any individual application. So as it runs there is a power management context for this thing that says, for example, here is my MP3 player and I know that when it’s running – now, it may be time shared – but when it is actually getting it slice of time then I don’t need this or that or I can shut off this. But I really need the memory controller working since that is how I’m getting my MP3 data.

LPD: Is it an estimator? Does it look at a number of calls to a certain memory unit?
Ready: It’s actually an engineered profile. It is not adaptive, it’s specified. In other words, you say that you know the device is on the machine or chip and you say, ‘I know that for my MP3 player because I understand it, I don’t use USB, and I’m not using BT.’ So when I am running every 5 milliseconds, or however often it gets scheduled out, it’s not a blanket profile. It’s dynamic. But depending upon what is running and the power profile associated with each of the individual applications, the power state of the machine is changed. So it’s trying to adapt the power consumption and presumably minimize it so it’s completely tailored to whatever activity you are doing and at a very fine level, such as process switching level.

Like anything else, there are a million tricks even beyond that to getting it to work. It gets very complex. With certain devices, if you don’t turn them on or off correctly they don’t work well. It’s typically not documented. Still, the net result we have seen in doing some battery operated devices, for example, is they had requirements that it run 18 hours. We came in double than that, so it can be engineered.

LPD Are you seeing more customers asking for low power?
Ready: It’s a big selling feature and a differentiator

LPD: One more question: You also support Google’s Android. This must be similar to your Atom and Moblin support.
Ready: Yes, exactly. Our insight was that Linux would be very important in mobile systems. One could argue that the first decade of doing that – at least before Moblin and Android – the core of what we had to do was around Linux – managing and comercializing that.

Well, Linux became so successful that the solution has moved up, as in the case of Moblin and Android. So for us, it’s more of the same. At first a customer had a choice of either buying commercial Linux or maintaining 6 million lines of code (the Linux kernel and tools). Now, Android comes out and the same situation occurs, except it is 60 million lines of code that is still in prototype form and still needs to be maintained. So the role of commercialization has gotten more important because the body of software they are being exposed to in prototype form is even larger.

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