Why didn't see ACPI: Preparing to enter system sleep state S5 in Ubuntu?

Question

I used Virtualbox to install two VMs, 1 CentOS 8, 1 Ubuntu 18.04.

After OS boot, I press "ACPI shutdown" in both VMs but got the different result:

CentOS 8:

[   42.562604] ACPI: Preparing to enter system sleep state S5
[   42.564738] reboot: Power down

Ubuntu 18.04:

         Starting Power-Off...
[   75.167832] reboot: Power down

We can see there is no ACPI-related string when Ubuntu shutting down, but both using the same version of Virtualbox, I think the BIOS is the same.

I can see this log in both VMs:

[    0.896151] ACPI: (supports S0 S5)

Can I say Ubuntu is not using ACPI ? I checked the grub of Ubuntu, it didn't assign acpi=off

What setting/config caused Ubuntu not to use ACPI?

Answer 1

Probably Ubuntu and CentOS are using different console log levels, so Ubuntu filters out the ACPI: Preparing... message and probably outputs a different message from the shutdown scripts instead.

Ubuntu's Starting Power-Off... message is not prefixed by [ <uptime in seconds> ] like all kernel log messages are, so it's probably produced by a different mechanism.

The log messages produced by the kernel are assigned to priority classes, and the low-priority messages are normally not displayed on the console. Please run sudo sysctl kernel.printk on both systems and see if the different distributions use different values. The command should output four numbers: the first one specifies the highest kernel message priority class to filter out.

Documentation on the kernel.printk values can be found here.