Updated: 28 September 2026 · Applies to: Ubuntu 24.04, Debian 12, AlmaLinux 9 and Rocky Linux 9
You can limit how many CPUs Linux uses, for example to test an application or for licensing. These steps work on Ubuntu 24.04, Debian 12, AlmaLinux 9 and Rocky Linux 9.
Note: On a Ucartz VPS the number of vCPUs comes from your plan. To get more or fewer, upgrade or change the plan in the client area instead.
Check the CPUs
nproc lscpu | grep -E "^CPU\(s\)|On-line"
At runtime (until reboot)
echo 0 > /sys/devices/system/cpu/cpu3/online # take CPU 3 offline echo 1 > /sys/devices/system/cpu/cpu3/online # bring it back
CPU 0 usually cannot be taken offline.
At boot (permanent)
nr_cpus=N limits the CPUs the kernel supports at all; maxcpus=N starts only N CPUs but lets you bring more online later.
grubby --update-kernel=ALL --args="nr_cpus=2" # AlmaLinux / Rocky # Ubuntu / Debian: add nr_cpus=2 to GRUB_CMDLINE_LINUX in /etc/default/grub, then: update-grub
Reboot and check with nproc. Remove it with grubby --update-kernel=ALL --remove-args="nr_cpus".
