linux-kernel - Re: [rfc][patch 2/3] x86: fix IO write barriers

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Date:	Thu, 4 Oct 2007 20:58:27 +0200
From:	Andi Kleen <ak@...e.de>
To:	Dave Jones <davej@...hat.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [rfc][patch 2/3] x86: fix IO write barriers

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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