posted on 2015-06-28 12:11:23
During the time of the APM standard (Advanced Power Management), power control was hardware-only:
/dev/apm, the kernel then made a bios call, hardware took control and set things straight.
To allow OS-directed power management, ACPI (Advanced Configuration and Power Interface) was created.
Now there exist state definitions in the ACPI standard for the
Generally the first state (ID 0) always defines a running/working mode without power savings or other power impediments.
G0 working S0 everything powered on like G0 - just monitor is turned off G1 sleeping S1 power on suspend (POS) - CPU not doing instructions, CPU caches cleared, CPU and ram fully powered, some devices turned off S2 CPU off - CPU caches are flushed to RAM, CPU powerless S3 standby / suspend to ram (STR) - only RAM remains powered on S4 hibernate / suspend to disk - RAM saved to HDD/SSD, and all power is turned off. G2 (S5) soft off - a little power to the motherboard, so the powerbutton or wake-on-lan etc. will work G3 mechanical off - powered off via powerswitch of PSU (power supply unit), so the power cord can be removed safely
D0 fully on - full power D1 intermediary state, definition depends on device D2 intermediary state, definition depends on device D3 off - unresponsive to the bus it is connected to D3 hot - has aux power, can assert power management request to transition to higher power states D3 cold - completely powered off
C0 operating state C1 halt - no execution is done, but can return to C0 in an instant via signals C2 stop clock - has to be awoken by hardware interrupts, external clocks run C3 sleep - cache is not maintained coherently, takes longer to wake up, no external clocks run. See next table, too.
From here, a non-exhaustive list of processor power states:
MODE NAME WHAT IT DOES CPUS C0 Operating-State CPU fully turned on All CPUs C1 Halt Stops CPU main internal clocks via software; 486DX4 and above bus interface unit and APIC are kept running at full speed. C1E Enhanced-Halt Stops CPU main internal clocks via software and reduces CPU voltage; All socket LGA775 CPUs bus interface unit and APIC are kept running at full speed. C1E Stops all CPU internal clocks. Turion 64, 65-nm Athlon X2 and Phenom CPUs C2 Stop-Grant Stops CPU main internal clocks via hardware; 486DX4 and above bus interface unit and APIC are kept running at full speed. C2 Stop-Clock Stops CPU internal and external clocks via hardware Only 486DX4, Pentium, Pentium MMX, K5, K6, K6-2, K6-III C2E Extended-Stop Grant Stops CPU main internal clocks via hardware and reduces CPU voltage; Core 2 Duo and above (Intel only) bus interface unit and APIC are kept running at full speed. C3 Sleep Stops all CPU internal clocks Pentium II, Athlon and above, but not on Core 2 Duo E4000 and E6000 C3 Deep-Sleep Stops all CPU internal and external clocks Pentium II and above, but not on Core 2 Duo E4000 and E6000; Turion 64 C3 AltVID Stops all CPU internal clocks and reduces CPU voltage AMD Turion 64 C4 Deeper-Sleep Reduces CPU voltage Pentium M and above, but not on Core 2 Duo E4000 and E6000 series; AMD Turion 64 C4E/C5 Enhanced-Deeper-Sleep Reduces CPU voltage even more and turns off the memory cache Core Solo, Core Duo and 45-nm mobile Core 2 Duo only C6 Deep-Power-Down Reduces the CPU internal voltage to any value, including 0V 45-nm mobile Core 2 Duo only
As can be seen here, intel defines up to 10 c-states, but this exceeds the ACPI specification.
The c-states can also be the source of time-sync errors in virtual machines. Since the hypervisor maybe cannot pass through the real-time clock, he may provide an emulated one, which in turn is related to the working cpu and its power states.
Energy consumption = no clock running = time halts
This can be the reason why your virtual machines are out of sync with the hypervisor clock every few days.
Disable the c-states in the BIOS, to make sure this will not happen, if your BIOS allows it.
Devices and CPU's running in D0 or C0 can have several p-states / performance states.
These are individually for the device / processor defined states in which the hardware can run:
P0 max power and frequency P1 less than P0 P2 less than P1 ... Pn less than P(n-1)
This is what i.e. intel SpeedStep is about, throttling CPU power and frequency to achieve energy savings.
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