FROM THE IGLOO

So, quick question – how many people know the difference between the various sleep states supported on most computer hardware?

ACPI defines several different power states that a computer can be in at any given time, below is a table copied from Wikipedia that details the sleep states on a computer:

State	Description
S0/Working	System is on. The CPU is fully up and running; power conservation operates on a per-device basis.
S1 Sleep	System appears off. The CPU is stopped; RAM is refreshed; the system runs in a low power mode.
S2 Sleep	System appears off. The CPU has no power; RAM is refreshed; the system uses a lower power mode than S1.
S3 Sleep (Standby)	System appears off. The CPU has no power; RAM is in slow refresh; the power supply is in a reduced power mode.
S4 (Hibernate)	System appears off. The hardware is completely off, but system memory has been saved as a temporary file onto the hard disk. This mode is also referred to as ‘Save To Disk’.
S5 Soft Off	System is off. The hardware is completely off, the operating system has shut down; nothing has been saved. Requires a complete reboot to return to the Working state.

Now as you go through these power states the computer consumes less and less power as you move down towards S5, at S5 most computers are pulling about 7watts of power to keep things like wake on lan possible. The biggest drop between the power states happens when you get into the S3 Sleep state, in a large number of cases that’s the sweet spot for saving power.

Now with Windows XP there are some problems with getting into the S3 sleep state, a lot of machines don’t have the S3 sleep state enabled by default as a number of vendors USB devices back when XP was released may not have supported going into a S3 sleep state, or would not be able to wake a computer from S3 sleep with the push of the mouse button or keyboard.

There’s a document on the Microsoft site that details this better than I can:

To prevent a negative end-user experience, Windows XP and Windows Server 2003 modify the power capabilities that are returned by the BIOS. Therefore, the system only supports wake by using the USB for the S1 system power state. If there is no USB device that is armed for wake from a system power state, the system enters the S3 system power state during standby.

The page also details a registry key that can be set that will force the computer to go into the S3 sleep state as opposed to defaulting to the S1 sleep state. So if you are looking to maximize the power savings on your older XP workstations – putting the following key into your system may be the trick to get the machines to sleep properly.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\usb
“USBBIOSx”=DWORD:00000000

Now some people may ask why not just go to S4, or S5 for that matter to save even more power? Well as you go deeper and deeper down the rabbit hole you wind up taking a longer and longer time to come back to the S0 (Working) state on the computer. Anybody that has worked with computers in the past knows that users who have a machine that has gone to sleep is not going to want to wait too long for a computer to come back online. Trick is to balance the power savings against the inconvenience for the end user.

Tags: S3 sleep power management Power Save