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    SubjectRe: [PATCH] make MADV_FREE lazily free memory
    Eric Dumazet wrote:
    > Rik van Riel a écrit :
    >
    >> Eric Dumazet wrote:
    >>
    >>> Rik van Riel a écrit :
    >>>
    >>>> Make it possible for applications to have the kernel free memory
    >>>> lazily. This reduces a repeated free/malloc cycle from freeing
    >>>> pages and allocating them, to just marking them freeable. If the
    >>>> application wants to reuse them before the kernel needs the memory,
    >>>> not even a page fault will happen.
    >>
    >>
    >>> I dont understand this last sentence. If not even a page fault
    >>> happens, how the kernel knows that the page was eventually reused by
    >>> the application, and should not be freed in case of memory pressure ?
    >>
    >>
    >> Before maybe freeing the page, the kernel checks the referenced
    >> and dirty bits of the page table entries mapping that page.
    >>
    >>> ptr = mmap(some space);
    >>> madvise(ptr, length, MADV_FREE);
    >>> /* kernel may free the pages */
    >>
    >>
    >> All this call does is:
    >> - clear the accessed and dirty bits
    >> - move the page to the far end of the inactive list,
    >> where it will be the first to be reclaimed
    >>
    >>> sleep(10);
    >>>
    >>> /* what the application must do know before reusing space ? */
    >>> memset(ptr, data, 10000);
    >>> /* kernel should not free ptr[0..10000] now */
    >>
    >>
    >> Two things can happen here.
    >>
    >> If this program used the pages before the kernel needed
    >> them, the program will be reusing its old pages.
    >
    >
    > ah ok, this is because accessed/dirty bits are set by hardware and not a
    > page fault.

    No it isn't.

    > Is it true for all architectures ?

    No.

    --
    SUSE Labs, Novell Inc.
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