C1 Halt State

[Aegis]

Two questions. What is it and how can I monitor if it's in effect?

[ripken204]

its a power saving mode for intel. i know that intel now has c1e halt state which is the same as amd's cool 'n queit.

[Aegis]

That can't be right. CnQ is equivalent to Intel's Speedstep, which are both P-States.

[ripken204]

from anandtech

C1E - Enhanced Halt State

"Whenever the OS executes the halt instruction, the CPU enters what is known as the halt state. Architecturally, what's going on in a halt state is the clock signal is shut off to the CPU for some period of time. With no clock signal, none of the logic in the chip will do anything and thus power consumption is reduced. Performance is also significantly reduced; however, the halt instruction isn't usually called during application usage, so the performance aspects of the halt state aren't very important.

The problem with the halt state is that it does nothing to reduce voltage, only current draw by stopping clocks from going to the CPU. Since Power varies linearly with both current and voltage (P = I * V), you're effectively only addressing half of the problem. The Enhanced Halt State, as Intel calls it, does two things: it reduces the clock speed of the CPU by decreasing the clock multiplier down to its minimum value (on the EE 965 series, that's 14x, or 2.8GHz), then reducing the voltage. The clock speed is reduced and then the voltage is dropped, to maintain stability.

Intel insists that the enhanced halt state is a significantly lower power state than the conventional halt state, thanks to the reduction in voltage in addition to the reduction in clock speed. While the standard halt state causes a linear reduction in power, Intel's enhanced halt state causes an exponential decrease in power, potentially offering better power savings than the standard halt state. The real world impact obviously depends on how idle your system happens to be."

[Aegis]

Is there any tool to monitor if it's working though?

[ripken204]

Is there any tool to monitor if it's working though?

i have no clue