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    SubjectRe: 'disposable' dirty pages [was: Out Of Memory in v. 2.1]
    On Fri, Oct 09, 1998 at 12:50:03PM -0400, Kenneth Albanowski wrote:
    > Yes, uniformly yucky. It feels like disappearing 4K pages could be of use
    > in some contexts (places where either the cached data fits easily in a 4K
    > page, or the data is continuous, and 4K chunks can be recalculated as
    > needed.) but not especially easy to use -- and also not portable. Just
    > move over to an Alpha, and you're working with 8K pages all of a sudden...

    See my suggestion for "unmap-on-discard" pages, that would solve this in
    a simpler way and has other useful properties (especially the ability to
    reclaim unused portions of cached objects). And no need for a VMA.

    > Yes, the only overhead is the extra (internal) trap to turn off the
    > discardable flag, and then the syscall to turn the bit back on (if you are
    > using the memory as a cache). That should be a miniscule overhead, though.

    You don't need the internal trap -- check if a page is discardable when
    it's aged enough to remove it. The normal page ageing will detect "next
    touch", you just have to clear the touched/dirty bits when marking a
    page discardable.

    Of course, architectures which don't have a touched bit per pte will
    need a trap, but they need it for page ageing anyway, it's presumably
    already done and implemented.

    -- Jamie

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