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I'm interested in theoretical limits, perhaps with examples of systems having huge numbers of CPU's.

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How many can it handle? or how many can it handle before you lose some benefit? also what kernel? I suspect that this answer changes somewhat for a computer patched to run a supercomputer. I seem to recall reading about a single instance using 4096 processors... – xenoterracide Dec 3 '10 at 10:43
what patch set, the normal patchset can't handle 4096 processors but linux has been patched to do it. (IIRC some answers seem to suggest it can) – xenoterracide Dec 3 '10 at 12:33
@xeno I think the fact that there's even a patchset that handles 4096 processors is stuff that should be mentioned on the Answer. – Tshepang Dec 3 '10 at 13:42
I don't recall much more than that, otherwise I'd give an answer, I also her performance gains pass 16? cores were limited... and that certain parts of the kernel needed a rewrite that had already begun. but really I don't have any citations and am not 100% that's why I'm not answering. – xenoterracide Dec 3 '10 at 14:41
up vote 14 down vote accepted

At least 2048 in practice. As a concrete example, SGI sells its UV system, which can use 256 sockets (2,048 cores) and 16TB of shared memory, all running under a single kernel. I know that there are at least a few systems that have been sold in this configuration.

According to SGI:

Altix UV runs completely unmodified Linux, including standard distributions from both Novell and Red Hat.

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this is what Launchpad has to say about Ubuntu, so I guess it applies to others:

1.Intel x86:
Maximum CPUs: 32 (including logical CPUs)
Maximum memory: 64GB
Maximum filesize: 8TB
Maximum filesystem size (ext3) 16TB
Maximum per-process virtual address space: 4GB

Maximum CPUs: 64
Maximum memory: 128GB
Maximum filesize: 8TB
Maximum filesystem size (ext3): 16TB
Maximum per-process virtual address space: N/A

These are standard max limitations whereas Linux cluster systems can scale up to 1024 CPU's.

That is 32 or 64 CPUs for x86 and x86_64 respectively.

Redhat says the same, but in a management-friendly table. Redhat EL6 can do 32 for x86, or 128 or 4096 CPUs cores for x86_64.

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arch/x86/Kconfig says these CONFIG_NR_CPUS limits can be raised if CONFIG_MAXSMP is enabled. – ephemient Dec 3 '10 at 18:30

This baby runs 10,368!

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I know this is an old post, but you're linking to a cluster computer. The question is about running a single kernel instance. – frodeborli Jul 31 '15 at 0:22

The x86_64 Linux kernel can handle a maximum of 4096 Processor threads in a single system image. This means that with hyper threading enabled, the maximum number of processor cores is 2048. Yes there is computers with more than 2048 processor cores; but these runs as clusters where several Linux kernels cooperate, connected with a high speed interconnect, typically an Infiniband fabric.

from the most current kernel 3.13, in ~/arch/x86/Kconfig :

config NR_CPUS

      This allows you to specify the maximum number of CPUs which this
      kernel will support.  If CPUMASK_OFFSTACK is enabled, the maximum
      supported value is 4096, otherwise the maximum value is 512.  The
      minimum value which makes sense is 2.

      This is purely to save memory - each supported CPU adds
      approximately eight kilobytes to the kernel image.
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