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    SubjectRe: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19
    On Wed, 2005-11-02 at 20:02 +1100, Nick Piggin wrote:
    > I agree. Especially considering that all this memory hotplug usage for
    > hypervisors etc. is a relatively new thing with few of our userbase
    > actually using it. I think a simple zones solution is the right way to
    > go for now.

    I agree enough on concept that I think we can go implement at least a
    demonstration of how easy it is to perform.

    There are a couple of implementation details that will require some
    changes to the current zone model, however. Perhaps you have some
    suggestions on those.

    In which zone do we place hot-added RAM? I don't think answer can
    simply be the HOTPLUGGABLE zone. If you start with sufficiently small
    of a machine, you'll degrade into the same horrible HIGHMEM behavior
    that a 64GB ia32 machine has today, despite your architecture. Think of
    a machine that starts out with a size of 256MB and grows to 1TB.

    So, if you have to add to NORMAL/DMA on the fly, how do you handle a
    case where the new NORMAL/DMA ram is physically above
    HIGHMEM/HOTPLUGGABLE? Is there any other course than to make a zone
    required to be able to span other zones, and be noncontiguous? Would
    that represent too much of a change to the current model?

    >From where do we perform reclaim when we run out of a particular zone?
    Getting reclaim rates of the HIGHMEM and NORMAL zones balanced has been
    hard, and I worry that we never got it quite. Introducing yet another
    zone makes this harder.

    Should we allow allocations for NORMAL to fall back into HOTPLUGGABLE in
    any case?

    -- Dave

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