I've recently realized just how similar mechanical and electronic engineering are when it comes to energy conservation. You see, I've recently begun construction of an energy-efficient, Frank-Lloyd Wright -inspired architect-designed house in Lakeway, Texas just outside Austin where the ARM US engineering team is based. We're coming to the end of a long and very hot summer here in Austin so naturally I spent quite an amount of time researching energy-efficient solutions to air-conditioning (A/C). To minimize energy consumption we're putting in an unusual heating and air-conditioning system which consists of eight ceiling cassettes all connected via refrigerant pipes to a single, external electronic inverter-controlled infinitely variable compressor. Each ceiling cassette A/C outlet can be individually controlled from an iPhone App from anywhere in the world and can run at any one of four speeds. Instead of pumping air around the house in ducts as with a traditional A/C system we pump refrigerant around in pipes, which is more efficient as it wastes less energy pumping. The total power output capacity of the eight outlets can total up to 130% of the maximum compressor output, but obviously you can't have all eight outlets on full blast at the same time.
In explaining the system to my friend and ARM colleague John Biggs he said "It's like the 'dark silicon' concept - you can build lots of transistors, but not have enough power to light them all up" which is an astute observation. And the analogy can be extended further which is exactly what I'm about to do. But before I do, I just want to take a moment more of your time to explain what 'Dark Silicon' is for those at the back of the class who weren't paying attention.
Mike Muller, CTO of ARM recently talked of 'Dark Silicon'. Moore's Law gives us twice the number of transistors every 2 years, but recently power hasn't scaled as well as transistor count. So the total number of transistors available is increasing, but the power per transistor no longer drops proportionately. If this continues then we'll end up in the era of 'Dark Silicon' where we can puts lots of transistors on the SoC, but we can't power them all at the same time.
I'm not sure about the term 'Dark Silicon' myself. It sounds like something from a science fiction novel, which we as all know is usually set hundreds of years in the future in a galaxy not so very far from our own. But I'd argue that to some extent we're in the era of dark silicon already. Today there are quad-core processor chips where the DVFS (dynamic voltage and frequency scaling) maximum clock speed is higher when two cores are running than all four, due to the total thermal dissipation of the package being reached. So you can have four moderate clock speed cores or two high clock speed cores, but you can't have all four at max speed at the same time. We haven't seen this sort of issue yet in the ARM world, but I'm willing to bet we'll get into this sort of thing within the next few years. Engineers are already thinking about it. In this case, the transistors are not completely powered off, just clocked down, so maybe 'Dark Silicon' isn't the right term? How about 'dimly-lit silicon' then? Sure, it doesn't sound as sexy, but at least it sounds like a real problem right now, and anyway, since when has engineering been sexy? Eventually so the argument goes, leakage will come to dominate, so you'll need to actually power down, not just under-clock and under-volt, which is when it truly becomes 'Dark Silicon'.
Going back to air-conditioning systems for a moment, it's very important that conventional systems are sized correctly. The reason is that they actually perform three separate functions simultaneously: Cooling, dehumidification and ventilation. Since all three happen at the same time it's important to size the system right as comfort is a factor of all three. If it's oversized you could get down to the set temperature too quickly and not have enough time for dehumidification and ventilation. Plus, it'll cycle on and off quickly which is inefficient as it takes a little time to ramp up to speed and then down again. If it's undersized then on hot day you might never get to the desired temperature. To ensure this sizing is done correctly, the Air Conditioning Contractors of America (ACCA) has a procedure for calculating the right-sized A/C unit called 'Manual-J'. This a requirement for a building permit (planning permission for our readers across the pond).
Manual-J is designed to make the calculation process simple and standardized for all contractors and all buildings. It assumes the outside air temperature will be 95F and the desired inside temperature is 75F. For each room in the house a 'heat load' is calculated in BTU/hr. BTU stands for 'British Thermal Unit' and is actually a unit of energy. So BTU/hr is a rate of energy movement, transfer or consumption, i.e. a unit of power, albeit a slightly odd one to those of us raised on SI units; and the A/C contractors probably don't even realize it. They calculate the heat loss power in BTU/hr for each room and then add them all up to create the overall house cooling-load (power), which just for kicks they change from BTU/hr to tons of cooling. (12000 BTU/hr = 1 ton of cooling). My system is 4 tons of cooling. Four tons of cooling is 14kW by the way, slightly more than the average SoC!
When I had the Manual-J done for my house, the contractor performing it proudly proclaimed my proposed system was undersized. 'You need 5 tons, not 4 tons! You might not be able to cool the whole house down to 75F on a hot day!'
'But this is a fully-zoned system' I explained. 'I have eight individual zones all controlled separately. Why would I want my bedrooms at 75F in the daytime? Why would I want my study cooled at night? Is it even 95F out at night? How can my three person family occupy all eight zones simultaneously?' And lastly, 'Have you done a full use-case analysis for the house? Then how do you know it's too small?' At this point he decided I was just an awkward customer and wanted nothing more to do with me. I suppose all that would be way too much trouble. Which is why Manual-J exists, but it's designed for a central air system and not for fully zoned systems.
Again, this is not so unlike the problems now facing the SoC architect. Traditionally, SoC power estimation has been performed by estimating power for each sub-block and then totaling them up to find the overall total worst-case power consumption. But is that worst-case a realistic scenario? I mean, just how much other stuff can you do on a smartphone when simultaneously making a voice call for example? You can do some things but I'd wager hard-core gaming isn't one of them. So why total the CPU and GPU power for hard-core gaming with the power for making a phone call? Because it's simpler, just like Manual-J. But it's probably not right, not for a 'fully-zoned' System-on-Chip with multiple independent power domains. Today's SoC architects need to take a far more sophisticated approach to power estimation. They need to list the use-case scenarios and figure out a different worst-case power number for each. It's time to call an end to the 'Manual-J' approach to SoC power estimation.
In part 2 I'll tell you about an even more sophisticated and radical approach to energy-efficient air conditioning that we designed for this house which will not get built, due to some limiting factors like the lack of availability of appropriate technology at reasonable cost and the small size of my bank account balance. And I'll explain how it's exactly like a radical, game-changing disruptive approach to energy management that ARM has actually developed which will change the whole landscape of power management in mobile devices forever. But unfortunately 'marcoms', those trendy, good-looking ARM people who wear square glasses and tell me what I can and can't say in these blogs reckon it's still supposed to be a bit hush-hush. So I'm sorry but I'll just have to leave you in suspense for a few more weeks until after 'the PR'. In the meantime, stay cool!
You could check on the progress of my house build on my blog at http://lakewayusonian.com to keep occupied while you wait...UPDATE:Energy Efficiency and Air Conditioning - Part 2: ARM Cortex-A7