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    SubjectRe: differences between MADV_FREE and MADV_DONTNEED
    Andrea Arcangeli wrote:
    > Now that MADV_REMOVE is in, should we discuss MADV_FREE?
    >
    > MADV_FREE in Solaris is destructive and only works on anonymous memory,
    > while MADV_DONTNEED seems to never be destructive (which I assume it
    > means it's a noop on anonymous memory).

    FWIW, in FreeBSD, MADV_DONTNEED is not destructive, and just makes pages
    (including anonymous ones) more likely to get swapped out.

    > Our MADV_DONTNEED is destructive on anonymous memory, while it's
    > non-destructive on file mappings.
    >
    > Perhaps we could move the destructive anonymous part of MADV_DONTNEED to
    > MADV_FREE?

    This would seem like the best way to go, since it would bring Linux's
    behavior more in line with what other systems do.

    > Or we could as well go relaxed and define MADV_FREE and MADV_DONTNEED
    > the same way (that still leaves the question if we risk to break apps
    > ported from solaris where MADV_DONTNEED is apparently always not
    > destructive).
    >
    > I only read the docs, I don't know in practice what MADV_DONTNEED does
    > on solaris (does it return -EINVAL if run on anonymous memory or not?).
    >
    > http://docs.sun.com/app/docs/doc/816-5168/6mbb3hrgk?a=view
    >
    > BTW, I don't know how other specifications define MADV_FREE, but besides
    > MADV_REMOVE I've also got the request to provide MADV_FREE in linux,
    > this is why I'm asking. (right now I'm telling them to use #ifdef
    > __linux__ #define MADV_FREE MADV_DONTNEED but that's quite an hack since
    > it could break if we make MADV_DONTNEED non-destructive in the future)

    FreeBSD's MADV_FREE only works on anonymous memory (it's a noop for
    vnode-backed memory), and marks the pages clean before moving them to
    the inactive queue, so that they can be freed or reused quickly, without
    causing a pagefault.

    -- Suleiman
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