linux-kernel - Re: [PATCH 0/4] i386 - pte update optimizations

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Message-ID: <p738xcw7942.fsf@bingen.suse.de>
Date:	13 Apr 2007 14:27:09 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Keir Fraser <keir@...source.com>
Cc:	Zachary Amsden <zach@...are.com>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...l.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Chris Wright <chrisw@...s-sol.org>,
	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hugh@...itas.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] i386 - pte update optimizations

Keir Fraser <keir@...source.com> writes:

> On 13/4/07 03:24, "Zachary Amsden" <zach@...are.com> wrote:
> 
> >> You do know that P6 and higher don't do locked bus references as long
> >> as the value is in the cache, right?
> > 
> > Yes.  Even then, last time I clocked instructions, xchg was still slower
> > than read / write, although I could be misremembering.  And it's not
> > totally clear that they will always be in cached state, however, and for
> > SMP, we still want to drop the implicit lock in cases where the
> > processor might not know they are cached exclusive, but we know there
> > are no other racing users.  And there are plenty of old processors out
> > there to still make it worthwhile.
> 
> LOCKed instruction suck really badly on the netburst microarchitecture (like
> factor of 10x, or not far off). I think it's probably because of their side
> effect of serialising memory accesses, causing horrible pipeline stalls.

Unfortunately they tend to be HyperThreaded usually (except for early ones 
and Celerons) and need the LOCK anyways.

-Andi
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