linux-kernel - Re: physical memory limit of 64-bit linux

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Message-Id: <200812171512.24993.ms@teamix.de>
Date:	Wed, 17 Dec 2008 15:12:15 +0100
From:	Martin Steigerwald <ms@...mix.de>
To:	linux-kernel@...r.kernel.org
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>
Subject: Re: physical memory limit of 64-bit linux


Thanks to all for the answers and the detailed discussions.

Am Dienstag, 16. Dezember 2008 schrieb Andi Kleen:
> On Tue, Dec 16, 2008 at 04:54:16PM +0100, Rafael J. Wysocki wrote:
> > On Tuesday, 16 of December 2008, Martin Steigerwald wrote:
> > > Hi!
> > >
> > > What is the physical memory limit for 64-bit Linux? I read about 40 bit
> > > address bus for AMD Athlon X2 (1 TB) and 48 bit for Barcelona X4 (256
> > > TB).
> > >
> > > Is 64-bit linux able to use that amount - provided that one would
> > > manage to build it into a machine? Or does it have a lower limit?
>
> It depends on which 64bit Linux; e.g. IA64 has larger limits.

Is it 50 bits (1024 TB)?

ms@...go:~/lokal/Kernel/linux-2.6.27/arch/ia64> 
egrep -r "(define.*MAX_PHYS_MEMORY|define.*MAX_PHYS_BITS|
define.*IA64_MAX_PHYSBITS)" *
include/asm/pgtable.h:#define IA64_MAX_PHYS_BITS        50      /* max. number 
of physical address bits (architected) */
include/asm/pgtable.h:#define _PAGE_PPN_MASK            (((__IA64_UL(1) << 
IA64_MAX_PHYS_BITS) - 1) & ~0xfffUL)
sn/kernel/setup.c:#define MAX_PHYS_MEMORY               (1UL << 
IA64_MAX_PHYS_BITS)     /* Max physical address supported */

Ciao,
-- 
Martin Steigerwald - team(ix) GmbH - http://www.teamix.de
gpg: 19E3 8D42 896F D004 08AC A0CA 1E10 C593 0399 AE90

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