Say goodbye to desktop lag while compiling your @world

Create a new slice

Start by creating a new systemd unit /etc/systemd/system/portage.slice:

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[Install]
WantedBy=slices.target

[Slice]
CPUShares=256

Enable and start the portage.slice unit you just created:

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systemctl enable --now portage.slice

CPUShares option defaults to 1024, systemd will create a user slice for each user with an active session, and all processes that user run will be assigned to that slice, anything that a user may run will receive 4 times the CPU time of processes assigned to the portage slice.

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➜  ~ cat /sys/fs/cgroup/cpu,cpuacct/user.slice/cpu.shares
1024

Repurpose PORTAGE_IONICE_COMMAND

PORTAGE_IONICE_COMMAND is those awesome variables you can set in your make.conf to alter how you build stuff, it should be a command string for portage to call to modify its own priority with a \${PID} placeholder that will be substituted with a pid. Maybe it was created with ionice in mind, but we can abuse that placeholder to write pids to the cgroup.procs file in the portage slice.

cgroup.procs file is present in every cgroup and contains a list of processes that are members of that particular cgroup. Writing a pid to this file will move all threads in that process at once to the cgroup. And that, is awesome :D

Add the following line to your /etc/portage/make.conf:

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PORTAGE_IONICE_COMMAND="sh -c \"echo \${PID} > /sys/fs/cgroup/systemd/portage.slice/cgroup.procs\""