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    Subject[PATCH] cacheline align pagevec structure
    Hi, 

    I commented this with Andrew before, we can shrink the pagevec structure to
    cacheline align it. It is used all over VM reclaiming and mpage
    pagecache read code.

    Right now it is 140 bytes on 64-bit and 72 bytes on 32-bit. Thats just a little bit more
    than a power of 2 (which will cacheline align), so shrink it to be aligned: 64 bytes on
    32bit and 124bytes on 64-bit.

    It now occupies two cachelines most of the time instead of three.

    I changed nr and cold to "unsigned short" because they'll never reach 2 ^ 16.

    I do not see a problem with changing pagevec to "15" page pointers either,
    Andrew, is there a special reason for that "16"? Is intentional to align
    to 64 kbytes (IO device alignment)? I dont think that matters much because
    of the elevator which sorts and merges requests anyway?



    Did some reaim benchmarking on 4way PIII (32byte cacheline), with 512MB RAM:

    #### stock 2.6.9-rc1-mm4 ####

    Peak load Test: Maximum Jobs per Minute 4144.44 (average of 3 runs)
    Quick Convergence Test: Maximum Jobs per Minute 4007.86 (average of 3 runs)

    Peak load Test: Maximum Jobs per Minute 4207.48 (average of 3 runs)
    Quick Convergence Test: Maximum Jobs per Minute 3999.28 (average of 3 runs)

    #### shrink-pagevec #####

    Peak load Test: Maximum Jobs per Minute 4717.88 (average of 3 runs)
    Quick Convergence Test: Maximum Jobs per Minute 4360.59 (average of 3 runs)

    Peak load Test: Maximum Jobs per Minute 4493.18 (average of 3 runs)
    Quick Convergence Test: Maximum Jobs per Minute 4327.77 (average of 3 runs)


    --- linux-2.6.9-rc1-mm4.orig/include/linux/pagevec.h 2004-09-08 16:13:14.000000000 -0300
    +++ linux-2.6.9-rc1-mm4/include/linux/pagevec.h 2004-09-08 16:48:51.703401288 -0300
    @@ -5,14 +5,14 @@
    * pages. A pagevec is a multipage container which is used for that.
    */

    -#define PAGEVEC_SIZE 16
    +#define PAGEVEC_SIZE 15

    struct page;
    struct address_space;

    struct pagevec {
    - unsigned nr;
    - int cold;
    + unsigned short nr;
    + unsigned short cold;
    struct page *pages[PAGEVEC_SIZE];
    };



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