Ubuntu – How many maximum CPUs does Ubuntu support by default


I think this is kernel dependent and probably will change over time depending on the kernel a release uses, correct me if wrong

I'd like to know two things for all the currently supported Ubuntu releases:

How many maximum CPUs can Ubuntu handle (by default) at the same time with a standard desktop kernel? Is that count different from a server kernel and 32bit/64bit systems?

Where can I find that information on my system?

Best Answer

Maximum CPUs (i.e. cores) supported by default:

Note: by 32-bit/64-bit we mean the common i386/x86_x64 architectures; PPC, ARM, etc. are not included.

12.04 LTS (and later):

  • Desktop/Server 32-bit: 8 cores/CPUs
  • Desktop/Server 64-bit: 256 cores/CPUs (but LiveCD supports 64 by default)

11.10 and below, including 10.04 LTS

  • Desktop/Server 32-bit: 8 cores/CPUs
  • Desktop 64-bit: 64 cores/CPUs
  • Server 64-bit: 256 cores/CPUs

How to find the maximum supported by your running kernel:

As @otus indicated, open a terminal with Ctrl-Alt-T, and type:

grep NR_CPUS /boot/config-`uname -r`

The below is a somewhat technical discussion on what maximum really means:

The "default" maximum is not the maximum!

  • You can recompile the desktop or server kernels to support up to 512 CPUs for 64-bit (8 is max for 32-bit).
  • but even those are not absolute maximums!:
    • The kernel has "experimental" options for going higher than the "default" maximum
    • The limit for 32-bit with BIGSMP=y is 512
    • The limit for 64-bit with MAXSMP=y is 4096 (or more!)
    • Just because the kernel supports so many cores on the general i386/x86_64 architecture does not mean your hardware will!
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