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    SubjectRe: [PATCH 2.5.43-mm2] New shared page table patch
    >> >> In many cases, this will stop the box from falling over flat on it's 
    >> >> face due to ZONE_NORMAL exhaustion (from pte-chains), or even total
    >> >> RAM exhaustion (from PTEs). Thus the performance gain is infinite ;-)
    >> >
    >> > So why has no one written a pte_chain reaper? It is perfectly sane
    >> > to allocate a swap entry and move an entire pte_chain to the swap
    >> > cache.
    >>
    >> I think the underlying subsystem does not easily allow for dynamic regeneration,
    >> so it's non-trivial.
    >
    > We swap pages out all of the time in 2.4.x, and that is all I was suggesting
    > swap out some but not all of the pages, on a very long pte_chain. And swapping
    > out a page is not terribly complex, unless something very drastic has changed.

    Right, it's swapping out the controlling structures without swapping
    out the pages themselves that's harder.

    >> IMHO, it's better not to fill memory with crap in the first place than
    >> to invent complex methods of managing and shrinking it afterwards. You
    >> only get into pathalogical conditions under sharing situation, else
    >> it's limited to about 1% of RAM (bad, but manageable) ... thus providing
    >> this sort of sharing nixes the worst of it. Better cache warmth on
    >> switches (for TLB misses), faster fork+exec, etc. are nice side-effects.
    >
    > I will agree with that if everything works so the sharing happens,
    > this is a nice feature.

    I think it will for most of the situations we run aground with now
    (normally 5000 oracle tasks sharing a 2Gb shared segment, or some
    such monster).

    >> The ultimate solution is per-object reverse mappings, rather than per
    >> page, but that's a 2.7 thingy now.
    > ???
    >
    > Last I checked we already had those in 2.4.x, and still in 2.5.x. The
    > list of place the address space is mapped.

    It's more complicated than that ... I'll let Rik or one of the K42
    guys who understand it better than I do explain it (yeah, I'm wimping
    out on you ;-))

    M.

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