Re: [rfc][patch 2/3] x86: fix IO write barriers
From: Dave Jones
Date: Thu Oct 04 2007 - 14:41:24 EST
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
> > > 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.
#define smp_mb() mb()
#define smp_rmb() rmb()
-#define smp_wmb() wmb()
+# define smp_wmb() wmb()
+# define smp_wmb() barrier()
> And we need memory barriers even without SMP
> when talking to device drivers. Only the smp_*b()s get noped
> on UP.
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