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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?

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similar question in Super User, the parameter is NR_CPUS I think. – Samik Jul 2 '12 at 8:47
up vote 15 down vote accepted

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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I found an Excellent Link here, but i am not sure about which kernel versions started supporting SMP and incrementing the CPU's , i hope you can include and make it a much detailed and better answer through this. Also SMP support started from Ubuntu 10.04 , as here. – atenz Jul 2 '12 at 12:12
@izx I have edited the title to reflect a more Ubuntu information approach. I am really looking for something that wee can redirect questions like "Does Ubuntu support my X processors system" to this one. – Bruno Pereira Jul 2 '12 at 12:14
@BrunoPereira, many thanks for clarifying -- I have updated to "top-post" the necessary info, while including the tech stuff at the bottom for those interested; I will continue to refine in the next few hours to be more newbie-friendly :) – izx Jul 2 '12 at 12:21
@izx pro! good job again. regards – Bruno Pereira Jul 2 '12 at 12:26

All I know for sure is that precise (12.04) desktop 64-bit -generic kernel supports maximum 256 cpus (by default).

You can find the number of CPUs your kernel config supports by doing:

grep CONFIG_NR_CPUS /boot/config-`uname -r`
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Since in 12.04 there is no longer a separate -server kernel, this applies to 64-bit Server systems as well. – Eliah Kagan Jul 2 '12 at 9:02

It all comes down to how one would define physical CPUs. Are they equivalent to sockets, so a multicore and/or hyperthreading CPU is counted as a single socket?

The Ubuntu kernel is configured to support 8 processors / cores in 32-bit and 64 processors / cores in 64-bit.1

As far as finding that information, I have used Sysinfo and I'm very happy with it. If you don't have it installed, you can do so by just pressing Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

sudo apt-get install sysinfo

Also to find the Number of processors in a system using terminal, just type:

dmesg |grep processor

I will keep researching this until I find the perfect answer.

1Source:Wiki Answers

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This goes to all releases that haven't reached it's EOL? – Bruno Pereira Jul 2 '12 at 8:51
This seems like outdated information. Starting with Ubuntu 12.04, the desktop and server kernels are the same. Many server machines have more than 8 processors/cores (and this has been the case for quite some time). It seems implausible that the single kernel, now used in both server and desktop systems, supports only up to 8 logical processors. – Eliah Kagan Jul 2 '12 at 8:52
@EliahKagan Is the number the same for 32, and 64 bit? – Mitch Jul 2 '12 at 9:18
@BrunoPereira, Mitch: No. I think it's 8 on 32-bit systems. I have a machine running Ubuntu 12.04 LTS 32-bit and the output of grep CONFIG_NR_CPUS /boot/config-$(uname -r) is CONFIG_NR_CPUS=8. There aren't too many 32-bit systems that run high-end servers or have more than 8 logical processors these days (though I expect that to change again once low-power systems become more parallel), so this makes sense. If I had info on all the non-EoL releases I'd post an answer, but I don't, at least not currently. Definitely feel free to include this info! – Eliah Kagan Jul 2 '12 at 9:23
@EliahKagan it was a typo. But if it 8 logical, then its 64 cores, right? – Mitch Jul 2 '12 at 9:30

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