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    SubjectRe: PAE/NX without performance drain?
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    Arjan van de Ven wrote:
    > On Sat, 2006-12-09 at 15:39 -0500, John Richard Moser wrote:
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    >> Apparently (as I've been told today) using a hardware NX bit in a 32-bit
    >> x86 kernel requires PAE mode. PAE mode is enabled with HIGHMEM64, which
    >> is (apparently) extremely slow.
    >
    >
    > it's not extremely slow.
    >
    > there is a minor performance delta, sure, but to be honest that's a
    > benchmark thing more than a real life thing.
    >
    > What did your measurements show that the slowdown was? And how did you
    > measure this?

    I didn't measure, I was told by various people on IRC. Also Google has
    some misleading facts:

    - Someone mentioned that HIGHMEM64G + 4G split costs 10%-30%. I
    immediately took that as "4G split might be friggin' expensive."

    - In some discussions Ingo mentioned HIGHMEM64G re RT preemption being
    a huge performance delta; but I haven't found evidence that this
    isn't a bug in RT.

    Too bad PAE can't be detected at boot time; someone else mentioned that
    some recent Pentium M laptops (and anything older than PPro) don't boot
    if PAE is on. Making 2 copies of the functions would be a pain in the
    ass; and using indirect addressing and function pointers would be...
    slow. In performance critical areas.

    I want my hardware NX bit working in Ubuntu without having to recompile
    my kernel dammit. How do I swing this one past them? The performance I
    can argue; breaking 586 and some random laptops (I've been contacted by
    someone who has several of these Pentium-M machines that die on PAE) I
    can't do.

    >
    >
    >
    >

    - --
    We will enslave their women, eat their children and rape their
    cattle!
    -- Bosc, Evil alien overlord from the fifth dimension
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