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    SubjectRe: [patch x86/core] x86: allow number of additional hotplug CPUs to be set at compile time
    On Thu, 2 Oct 2008 21:44:09 +0200
    Andi Kleen <andi@firstfloor.org> wrote:

    > On Thu, Oct 02, 2008 at 03:25:21PM -0400, Chuck Ebbert wrote:
    > > On Thu, 02 Oct 2008 11:12:51 +0200
    > > Andi Kleen <andi@firstfloor.org> wrote:
    > >
    > > > Chuck Ebbert <cebbert@redhat.com> writes:
    > > >
    > > > > The default number of additional CPU IDs for hotplugging is determined
    > > > > by asking ACPI or mptables how many "disabled" CPUs there are in the
    > > > > system, but many systems get this wrong so that e.g. a uniprocessor
    > > > > machine gets an extra CPU allocated and never switches to single CPU
    > > > > mode.
    >
    > What do you mean with single cpu mode?
    >
    > e.g. the lock prefix rewriting is only determined by online CPUs,
    > not possible CPUs. And this only affects the possible ones.
    >


    The prefix rewriting doesn't happen unless I boot with additional_cpus=0,
    maxcpus=1, or with this patch applied and the config option set. I think
    the rules for when/if the rewriting happens changed a while ago to avoid
    multiple switches and now it's not happening at all on this machine by
    default.

    Oh, and with NR_CPUS=512 I am seeing 1.6MB per-cpu data (I'll have to
    check that, but I remember being surprised at how big the number was.)


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