3 ms·

Yes, this is also part of Servo's strategy apparently: spread work across multiple cores, but each core does less and can be more efficient in terms of power dr

by ehsanu1 13y ago

Yes, this is also part of Servo's strategy apparently: spread work across multiple cores, but each core does less and can be more efficient in terms of power draw. Faster + less battery use in one fell swoop. Though it's not really clear to me why this actually results in an efficiency win over maxing out a single core.

pcwalton 13y ago
The way I've heard it, as a very imprecise rule of thumb, increasing clock speed scales power consumption quadratically, while adding CPUs increases power consumption linearly. So, very roughly speaking, one 1.6 GHz CPU uses twice as much power as two 800 MHz CPUs.
- cwzwarich 13y ago
  The equation for CMOS switching power (as opposed to leakage power) is capacitance * voltage^2 * frequency but the voltage limits the clock frequency. The exact scaling of maximum clock frequency with voltage depends on the circuit and the process, but from taking a glance at the voltage tables it looks like the main cores in current Snapdragons are generally running in the 0.8-1.2v range across their entire frequency range.
- Symmetry 13y ago
  Most of the power differences between cores these days isn't in terms of clock speeds but in terms of extra structures that cause more instruction to be executed each cycle. The rule of thumb involving voltage/speed scaling is that your power use is indeed the square of your performance more or less when you're in a reasonable region. However, a relatively simple in-order core like an ARM A7 might also only take 1/4 the energy to execute a given instruction of a complex out-or-order core like an ARM A15 even when both are clocked at 1 GHz on the same process.
auvrw 13y ago
> Though it's not really clear to me why this actually results in an efficiency win over maxing out a single core. i heard the whole multiple core thing is spurred in part by manufacturers trying to increase fab yield: if one of the cores on a multi-core chip is bad, it can still be sold, albeit at a lower price, rather than thrown out.

by ehsanu1 13y ago

pcwalton 13y ago

The way I've heard it, as a very imprecise rule of thumb, increasing clock speed scales power consumption quadratically, while adding CPUs increases power consumption linearly. So, very roughly speaking, one 1.6 GHz CPU uses twice as much power as two 800 MHz CPUs.

cwzwarich 13y ago
The equation for CMOS switching power (as opposed to leakage power) is capacitance * voltage^2 * frequency but the voltage limits the clock frequency. The exact scaling of maximum clock frequency with voltage depends on the circuit and the process, but from taking a glance at the voltage tables it looks like the main cores in current Snapdragons are generally running in the 0.8-1.2v range across their entire frequency range.
Symmetry 13y ago
Most of the power differences between cores these days isn't in terms of clock speeds but in terms of extra structures that cause more instruction to be executed each cycle. The rule of thumb involving voltage/speed scaling is that your power use is indeed the square of your performance more or less when you're in a reasonable region. However, a relatively simple in-order core like an ARM A7 might also only take 1/4 the energy to execute a given instruction of a complex out-or-order core like an ARM A15 even when both are clocked at 1 GHz on the same process.

auvrw 13y ago

> Though it's not really clear to me why this actually results in an efficiency win over maxing out a single core. i heard the whole multiple core thing is spurred in part by manufacturers trying to increase fab yield: if one of the cores on a multi-core chip is bad, it can still be sold, albeit at a lower price, rather than thrown out.