From owner-freebsd-current Fri Feb 1 13:50:49 2002 Delivered-To: freebsd-current@freebsd.org Received: from mailg.telia.com (mailg.telia.com [194.22.194.26]) by hub.freebsd.org (Postfix) with ESMTP id AB7D237B43B for ; Fri, 1 Feb 2002 13:50:17 -0800 (PST) Received: from d1o913.telia.com (d1o913.telia.com [195.252.44.241]) by mailg.telia.com (8.11.6/8.11.6) with ESMTP id g11LoAA16650 for ; Fri, 1 Feb 2002 22:50:10 +0100 (CET) Received: from falcon.midgard.homeip.net (h185n2fls20o913.telia.com [212.181.163.185]) by d1o913.telia.com (8.8.8/8.8.8) with SMTP id WAA15839 for ; Fri, 1 Feb 2002 22:50:08 +0100 (CET) Received: (qmail 50144 invoked by uid 1001); 1 Feb 2002 21:50:06 -0000 Date: Fri, 1 Feb 2002 22:50:06 +0100 From: Erik Trulsson To: =?iso-8859-1?Q?G=E9rard?= Roudier Cc: Jason Evans , Kenneth Culver , "Cameron, Frank" , Terry Lambert , David Malone , "'freebsd-current@freebsd.org'" Subject: Re: AMD AGP Bug Message-ID: <20020201215006.GA50090@student.uu.se> Mail-Followup-To: =?iso-8859-1?Q?G=E9rard?= Roudier , Jason Evans , Kenneth Culver , "Cameron, Frank" , Terry Lambert , David Malone , "'freebsd-current@freebsd.org'" References: <20020131173408.B63502@canonware.com> <20020131211810.B1769-100000@gerard> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: <20020131211810.B1769-100000@gerard> User-Agent: Mutt/1.3.25i Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG On Thu, Jan 31, 2002 at 09:32:48PM +0100, G=E9rard Roudier wrote: >=20 >=20 > On Thu, 31 Jan 2002, Jason Evans wrote: >=20 > > On Wed, Jan 30, 2002 at 11:14:48PM +0100, G=E9rard Roudier wrote: > > > > > > Linux can be fixed, but the useless writes of the existing Athlons fr= om > > > the very fast cache to the relatively very slow memory cannot. And all > > > Athlon users may well pay this penalty under any OS... unless we wan= t to > > > disable caching. :) > > > > Have you done benchmarks to show that the speculative writes are useless > > often enough to cause enough memory bus contention that overall perform= ance > > is degraded, despite the speedups when the speculative writes are valid? >=20 > I haven't done any benchmark of this sort, neither intend to do any since > I haven't time for that. But I wrote in my email that my 2 Athlon systems > worked fine and fast, just to indicate that for normal use I didn't see > any performance problem at all. >=20 > > I > > suspect that AMD in fact performed such tests; otherwise they wouldn't = have > > gone to the trouble of implementing speculative writes. >=20 > The Athlon rewriting same value to cacheable memory under the knees of > programmers looks a severe issue to me if it is true. Not only AGP memory > can be affected. What about SMP, MMIO (if some cacheable mapping exists), > etc...? I am not familiar with the acronym MMIO is so I can't comment on that.=20 In general though, having memoryspace used for memory-mapped I/O devices (including AGP) marked as cacheable is a bad idea unless you are very careful and know exactly what you are doing. For SMP it shouldn't be any problem. Multi-CPU systems normally run some cache-coherence protocol between themselves to make sure that things like this is not a problem. >=20 > In my opinion, OSes having some cacheable mapping to AGP memory is not the > real problem. Just it has revealed the AMD issue. It might be argued that there should be some cache-coherence protocol between the CPU and the AGP device. Not knowing how AGP is specified I don't know if this interaction between the CPU and AGP is a bug or just working as specified. I suspect it is the latter though. --=20 Erik Trulsson ertr1013@student.uu.se To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message