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    SubjectRe: [PATCHSET] percpu: generalize first chunk allocators and improve lpage NUMA support
    On Tue, Jun 30, 2009 at 03:39:52PM -0400, Christoph Lameter wrote:
    > On Tue, 30 Jun 2009, Ingo Molnar wrote:
    >
    > > Yeah, it's a bug for something like a virtual environment which
    > > boots generic kernels that might have 64 possible CPUs (on a true
    > > 64-way system), but which will have fewer in practice.
    >
    > A machine (and a virtual environment) can indicate via the BIOS tables or
    > ACPI that there are less "possible" cpus. That is actually very common.
    >
    > The difference between actual and possible cpus only has to be the number
    > of processors that could be brought up later. In a regular system that is
    > pretty much zero. In a fancy system with actual hotpluggable cpus there
    > would be a difference but usually the number of hotpluggable cpus is
    > minimal.

    A hypervisor running on a 4-socket Beckton platform will have 32
    (without hyperthreading) or 64 (with hyperthreading) CPUs to
    allocate 1Q2010. A hypervisor supporting ACPI hot-plug would be able
    then to hot-plug from one to 64 cores into a single linux guest if it
    wanted to (and didn't overcommit, so gang-scheduling wasn't required).

    Such a hypervisor might typically pass the maximum number of
    CPUs in via the guest ACPI tables when the guest boots, to provide
    the maximum flexibility in being able to grow and shrink the guest,
    with only a subset marked as initially present.

    I've been working recently with a 128 (shanghai) / 192 (istanbul)
    core shared memory system which supports hot plugging CPUs into Linux,
    and the feature would be less useful if a hot-plug event could fail
    because the per-cpu area couldn't be allocated at the time of the
    event (which argues for pre-allocation, if a large physically contiguous
    region is required).

    That said, I don't see much use for 32-bit kernels in environments with
    such CPU counts.

    scott


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