Systemd vs Nice Level – Understanding CPUShares Property

Ah, it's not the systemd-logind feature where each user gets it's own cgroup. I think the change responsible here is older; they're just confusingly similar. (I searched "process group fair scheduling", thinking it might be something based on unix's "process groups" that I never really understand). Wikipedia:

The Linux kernel received a patch for CFS in November 2010 for the 2.6.38 kernel that has made the scheduler fairer for use on desktops and workstations.

When a task calls __proc_set_tty(), the process wide reference to the default group is dropped, a new task group is created, and the process is moved into the new task group. Children thereafter inherit this task group, and increase its refcount. On exit, a reference to the current task group is dropped when the last reference to each signal struct is dropped. The task group is destroyed when the last signal struct referencing it is freed. At runqueue selection time, IFF a task has no cgroup assignment, its current autogroup is used.

The feature is enabled from boot by default if CONFIG_SCHED_AUTOGROUP is selected, but can be disabled via the boot option noautogroup, and can be also be turned on/off on the fly [via /proc/sys/kernel/sched_autogroup_enabled: Writing 0 there disables it for newly created tasks, writing 1 enables it.]

The primary issues solved by this are for multi-core as well as multi-cpu (SMP) systems experiencing increased interactive response times while performing other tasks that use many threads in those tasks. A simple explanation is that one will be able to still watch a video, read email and perform other typical desktop activities without glitches or choppiness while compiling the Linux kernel or a similar process such as encoding video. However, there are objections to this statement.

Main PID is the PID of the process that is started using ExecStart= options (in the example sshd). The Process: is used for any other process that might have been started by the service using ExecStartPre, ExecStartPost, ExecStopPost, etc options. (in this example, sshd-keygen has been started using the ExecStartPre= option). You can have multiple lines on Process: if there are more than one processes started by the service, other than the main process. If there is no other process involved, you will only see the Main PID value.

I admit this is not very clear in the docs for ExecStartPre but the doc for ExecStart clarifies that the main process is the one started by that option, except for Type=forking, in which case, the daemon process is considered the main process.