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    Subjectresults: Remove 8 bytes from struct page on 64bit archs

    Hi,

    > I'm curious to see how large the tradeoff is with calculating
    > virtual in page_address(). The overhead there may be larger than
    > the win we get from better cacheline footprint in struct page

    Check out:

    http://samba.org/~anton/linux/struct_page/

    "struct page can be hazardous to your health"

    On this machine (2 way power3) we see a ~5% improvement on dbench by
    removing ->zone. Quite a nice improvement.

    When we also remove ->virtual and use pointer arithmetic to do
    page_address() performance drops back down. The reason for this can
    be found in:

    http://samba.org/~anton/linux/struct_page/2.4.18pre1-nopagezone-nopagevirtual/2/6.html

    Search for divd, remembering the percentage to the left is shifted one
    instruction down. Yes divides really hurt.

    Basically the difference is:


    page_address() using page->virtual:

    ld 25,112(31)


    page_address() using pointer arithmetic:

    .LC1:
    .quad mem_map
    .LC2:
    .quad 0xc000000000000000

    ...

    ld 9,.LC1
    li 11,120 ; sizeof(struct page)
    ld 10,.LC2
    ld 0,0(9)
    subf 0,0,31
    divd 0,0,11
    sldi 0,0,12
    add 25,0,10


    Perhaps the compiler should be optimising this better (can we replace
    the divide?)

    Anton
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