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    SubjectRe: [rfc][patch 2/3] x86: fix IO write barriers
    Date
    On Thursday 04 October 2007 20:41:07 Dave Jones wrote:
    > On Thu, Oct 04, 2007 at 08:21:59PM +0200, Andi Kleen wrote:
    > > On Thursday 04 October 2007 20:10:44 Dave Jones wrote:
    > > > On Thu, Oct 04, 2007 at 07:53:16PM +0200, Andi Kleen wrote:
    > > > >
    > > > > > The only vendor that ever implemented OOSTOREs was Centaur, and they
    > > > > > only did in the Winchip generation of the CPUs. When they dropped it
    > > > > > from the C3, I asked whether they intended to bring it back, and the
    > > > > > answer was "extremely unlikely".
    > > > > >
    > > > >
    > > > > Do you know if it made a big performance difference?
    > > >
    > > > On the winchip, it was a huge win. I can't remember exact numbers,
    > > > but pretty much every benchmark I threw at it at the time showed
    > > > significant improvement.
    > >
    > > Significant as in >10%?
    >
    > "Worth about 10-20% performance" according to the 2.4.18pre9-ac4
    > release notes: http://www.linuxtoday.com/news_story.php3?ltsn=2002-02-14-015-20-NW-KN

    Are there numbers for a newer kernel available too?

    > > > > But yes we should probably just remove this special case to make
    > > > > maintenance easier.
    > > > It's CONFIG_SMP anyway, which none of the winchips were.
    > >
    > > It's not.
    >
    > You're right it isn't now, but Nicks patch seems to change it so that it is.
    >
    > ...
    >
    > #ifdef CONFIG_SMP
    > #define smp_mb() mb()
    > #define smp_rmb() rmb()
    > -#define smp_wmb() wmb()
    > +#ifdef CONFIG_X86_OOSTORE
    > +# define smp_wmb() wmb()
    > +#else
    > +# define smp_wmb() barrier()
    > +#endif

    That is only for smp_wmb() which are always SMP only

    -Andi
    -
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