Ubuntu – In Ubuntu 16.04, which ACPI Global State does Suspend result in


I have read the post on "What is suspend".

However, I would like to know exactly which ACPI Global State does pressing the Suspend button lead to. Can someone explain this?

Specifically, how do I know if pressing the Suspend button in Ubuntu 16.04 will lead to S1, S2, S3, S4, or S5? Is this determined by setting in the UEFI/BIOS or Ubuntu or both?

Best Answer

It depends on Ubuntu (and the BIOS capabilities). It couldn't be any other way because the operating system needs to coordinate a great deal of things to enter any state above S2. The BIOS may however inform the operating system that it would like to enter a particular state.

There are two suspension modes in Ubuntu, Linux and modern PCs in general:

  • Suspend-to-RAM (ACPI S3) powers off the CPU and all other devices unless they indicate that they need to stay on in S3. The main memory (RAM) retains its power supply and maintains the system state for later resumption.

  • Suspend-to-disk or "hibernate" (ACPI S4) saves the system state to a non-volatile storage medium, typically a hard drive or SSD, and powers off the system entirely. The operating system can see the stored system image on reboot and choose to resume from it.

    It's not enabled or exposed to Ubuntu desktop users by default these days.

The other ACPI global states you mention are:

  • S1 and S2 are states with less power saving potential but quicker resumption. These are not typically considered suspension state but a computer may choose to enter them automatically as soon as the CPU is unused for a few milliseconds. This is typically done by the CPU and BIOS without direct interaction of the operating system.

  • Power-off (S5) means exactly what you'd think it does.


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