disabling cpu cores on quad core processor on linux

Question

I want to disable 3 CPU cores and run my processor on a single core. I have used command:maxcpus=1. But after this I executed this command ls /sys/devices/system/cpu. It still shows cpu0,cpu1,cpu2,cpu3.

I also tried :echo 0 > /sys/devices/system/cpu3/online but I get the following error: no such file or directory.

Answer 1

As Patrick has indicated in a comment, you got the path under /sys wrong.

echo 0 > /sys/devices/system/cpu/cpu3/online

If you want to switch all CPUs off except cpu0:

for x in /sys/devices/system/cpu/cpu[1-9]*/online; do
  echo 0 >"$x"
done

---

Typing maxcpus=1 at a shell prompt has no effect. More precisely, it sets the variable maxcpus to the value 1 in that shell, which doesn't have any other effect. You can set the number of CPUs at boot time by passing maxcpus as a kernel parameter. For that, you have to change your bootloader configuration (e.g. to change the kernel command line in U-Boot).

Answer 2

you do not necessarily turn off or disable cores.

you would use cpusets and taskset

http://man7.org/linux/man-pages/man7/cpuset.7.html

A cpuset defines a list of CPUs and memory nodes...

The cpuset filesystem is a pseudo-filesystem interface to the kernel cpuset mechanism, which is used to control the processor placement and memory placement of processes. It is commonly mounted at /dev/cpuset.

On systems with kernels compiled with built in support for cpusets, all processes are attached to a cpuset, and cpusets are always present. If a system supports cpusets, then it will have the entry nodev cpuset in the file /proc/filesystems. By mounting the cpuset filesystem (see the EXAMPLE section below), the administrator can configure the cpusets on a system to control the processor and memory placement of processes on that system. By default, if the cpuset configuration on a system is not modified or if the cpuset filesystem is not even mounted, then the cpuset mechanism, though present, has no effect on the system's behavior.

The CPUs of a system include all the logical processing units on which a process can execute, including, if present, multiple processor cores within a package and Hyper-Threads within a processor core. Memory nodes include all distinct banks of main memory; small and SMP systems typically have just one memory node that contains all the system's main memory, while NUMA (non-uniform memory access) systems have multiple memory nodes.

In short, if you have 1 cpu having 6 cores you would configure cpusets and launch your process in a cpuset that is configured on just one core, say core #3 for example. If it was a parallel process it would all be confined to that one core such that if you launched 4 processes in a given cpuset having just one core defined, then each of the 4 processes would get 25% cpu utilization on core #3.

Building off of that, what typically happens is a cpuset is configured such that

• in a 200+ core system for example, cpusetA is cores 0..60 wherever those might be located, cpusetB is cores 61..70; cpusetC is cores 71..80; and so on however an admin/architect chooses to configure.
• cpusetA is allocated to certain users and/or specific software programs; cpusetB is allocated to different users/programs; and so on.
• a user launches a job (process) which would request N cores... within a given cpuset, and now those multiple (parallel) processes are confined to that given cpuset. And for those N parallel confined to a given cpuset, each of those processes would/should make use of processor affinity or cpu affinity so those parallel processes do not thrash around on different cores within the cpuset.

also: https://linux.die.net/man/1/taskset