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    SubjectRe: Q: PREFETCH_STRIDE/16
    >>>>> On Thu, 23 May 2002 09:34:16 -0700 (PDT), "David S. Miller" <davem@redhat.com> said:

    DaveM> I mentioned this 1 or 2 days ago in the TLB thread with
    DaveM> Linus, you pessimistically maintain a tiny bitmap per
    DaveM> mm_struct which keeps track of where mappings actually are.
    DaveM> You use some hash function on the virtual address to
    DaveM> determine the bit. You clear it when the mm_struct is new,
    DaveM> and you just set bits when mappings are installed. Very
    DaveM> simple.

    DaveM> Then all of these "walk all valid page tables" loops that
    DaveM> scan entire mostly empty pages of pgd/pmd/pte entries for no
    DaveM> reason can just check the bitmap instead.

    DaveM> Most of the exit overhead is in clear_page_tables walking
    DaveM> over entire pages. It effectively flushes the cache unless
    DaveM> all you are doing is fork/exit/fork/exit

    Sounds like something worth experimenting with. I doubt you could
    really avoid (effectively) flushing the caches, but even if there are
    just a few zero bits in the bitmap at the time of the tear-down, a
    fair amount of time could be saved.

    --david
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