Shouldn't every idle core be in low power mode? Doesn't the idle instruction (hlt) tell the cpu to power parts down? I thought it was like a switch, you don't

Shouldn't every idle core be in low power mode? Doesn't the idle instruction (hlt) tell the cpu to power parts down? I thought it was like a switch, you don't need light? Flip it off, need some? Turn it on at nearly no cost. There might be newer multistage power down but to wear level and use all cores, this is the right algorithm.