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There's No Such Thing As a General-Purpose Processor
CowboyRobot writes: David Chisnall of the University of Cambridge argues that despite the current trend of categorizing processors and accelerators as "general purpose," there really is no such thing and believing in such a device is harmful.
"The problem of dark silicon (the portion of a chip that must be left unpowered) means that it is going to be increasingly viable to have lots of different cores on the same die, as long as most of them are not constantly powered. Efficient designs in such a world will require admitting that there is no one-size-fits-all processor design and that there is a large spectrum, with different trade-offs at different points."
"The problem of dark silicon (the portion of a chip that must be left unpowered) means that it is going to be increasingly viable to have lots of different cores on the same die, as long as most of them are not constantly powered. Efficient designs in such a world will require admitting that there is no one-size-fits-all processor design and that there is a large spectrum, with different trade-offs at different points."
This seems to be how the human brain works, and it runs on less than 100 watts (100 watts corresponds to 2000 Calories per day).
A whole woman consumes 100 watts. Of that brain is about 20 watts. Also, watt represents momentary consumption and calories are a fixed mass of energy, so you can't directly compare them.
100 watts corresponds to 2000 Calories per day
Also, watt represents momentary consumption and calories are a fixed mass of energy
Calories and calories per day are not the same unit. A calorie is 4.18 kJ, and a calorie per day is 4.18 kJ / 86.4 ks = 48.4 mW. Multiply this by 2000 and you'll end up very close to 100 W.
Because it is inefficient. In addition to higher energy bills, a less efficient architecture means shorter battery life in a mobile device, more noise in a desktop PC, and fewer servers per rack in a datacenter.
There are "general purpose" microcontrollers that use microWatts of power. That can run on one tiny watch battery for years.
For example, http://www.microchip.com/wwwpr...
From datasheet,
* 30 microAmps per Mhz
* 20 nanoAmps in sleep
So a 100mAh 3V watch battery would last 570 years on sleep mode and 3-5 months operating at 1MHz. Or at 31kHz, with some sleep it should operate for years on a button cell.
And it's a fully programmable, general purpose microcontroller.
So what's the problem? Too inefficient?
I'm the author of TFA. There's a big difference between a general purpose processor and a general purpose computer. A lot of current research in computer architecture is focussed on the idea that you have a sharp divide between accelerators and general purpose CPUs. The point of the article is that different CPU microarchitectures are specialised for different workloads (one of the cited results was that in a big.LITTLE arrangement, the A7 core runs one of the SPEC benchmarks faster than the A15 because of its lower cache access time, for example) and that there are a lot of assumptions about the kind of code that the general purpose core will run. Many of these are true for C code, but a lot less true for code written in other languages. The communication patterns that mainstream multicore processors are optimised for are heavily tied to C, to the extent that if you have a language with a shared-nothing abstraction and message passing then the only way of implementing it is horrendously inefficient at the hardware level.
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