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    SubjectRe: RFC: remove __read_mostly
    On Thu, Dec 13, 2007 at 11:48:27PM +0100, Eric Dumazet wrote:
    > Adrian Bunk a écrit :
    >> I tried the following patch with a full x86 .config [1]:
    >> --- a/include/asm-x86/cache.h
    >> +++ b/include/asm-x86/cache.h
    >> -#define __read_mostly __attribute__((__section__(".data.read_mostly")))
    >> +/* #define __read_mostly __attribute__((__section__(".data.read_mostly"))) */
    >> The result [2,3] was:
    >> -rwxrwxr-x 1 bunk bunk 46607243 2007-12-13 19:50 vmlinux.old
    >> -rwxrwxr-x 1 bunk bunk 46598691 2007-12-13 21:55 vmlinux
    >> It's not a surprise that the kernel can become bigger when __read_mostly
    >> gets used, especially in cases where __read_mostly prevents gcc
    >> optimizations.
    >> My question is:
    >> Is there anywhere in the kernel a case where __read_mostly brings a
    >> measurable improvement or can it be removed?
    > Yes, there are many cases where read_mostly brings huge improvements.
    > I did test your idea on a 4 CPUS server, and system time was roughly
    > doubled, from 11% to 20%

    Thanks, that answered my question.

    > Of course, you noticed that puting a __read_mostly attribute force the
    > linker to reserve space for the variable. So a null variable previously in
    > bss section (no space in vmlinux file) is now in .data.read_mostly. Not a
    > big deal.
    > If you want, you could play some .lds games to create sort of a
    > "bss.read_mostly" section to save 10000 bytes in vmlinux.

    No, what I was thinking of kernel code like

    static int fooxxxvar = 0;
    static int __read_mostly fooyyyvar = 0;

    void mytest()
    if (fooxxxvar)

    if (fooyyyvar)

    If you run "strings file.c | grep foo" on the file it will only return
    "fooyyy" but not "fooxxx".

    It's not a big deal, but it's an area where __read_mostly affects gcc
    and not ld.



    "Is there not promise of rain?" Ling Tan asked suddenly out
    of the darkness. There had been need of rain for many days.
    "Only a promise," Lao Er said.
    Pearl S. Buck - Dragon Seed

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