Date: Tue, 09 Nov 2010 11:17:08 -0800 From: Julian Elischer <julian@freebsd.org> To: Alan Cox <alc@rice.edu> Cc: alc@freebsd.org, sbruno@freebsd.org, FreeBSD Current <current@freebsd.org> Subject: Re: limits to memory on amd64 Message-ID: <4CD99E34.10002@freebsd.org> In-Reply-To: <4CD98BE7.7030208@rice.edu> References: <4CD97A9A.8000007@freebsd.org> <20101109170453.942D95B89@mail.bitblocks.com> <4CD982E9.70500@freebsd.org> <4CD98BE7.7030208@rice.edu>
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On 11/9/10 9:59 AM, Alan Cox wrote: > Julian Elischer wrote: >> On 11/9/10 9:04 AM, Bakul Shah wrote: >>> On Tue, 09 Nov 2010 08:45:14 PST Julian >>> Elischer<julian@freebsd.org> wrote: >>>> During the discussion at MeetBSD the question came up as to what >>>> the real >>>> limiting factors were with regard to how much RAM a system could >>>> have. >>>> it was put to us that the limit was currently around 512 GB, >>>> though no-one >>>> at teh discussion knew what the mechanism of the limitation was or >>>> what might ligh beyond it. >>>> >>>> Could anyone who knows, pipe upt and let use know what the >>>> factors are, >>>> and if the current limit is overcome, what the next one after >>>> that will be? >>> You mean beyond architectural limits? >> >> no, though of course they are relevant. >> I was thinking more of details like limits to the KVM space or >> any limitations there may be on the size of the direct-map region, >> or maybe some limit on some data structure size in the kernel. >> Since I don 't know the details, this is exactly the question.. >> what IS the limit? > > The changes to support more than 512GB RAM should be > straightforward. Off the top of my head, it will require some > constant definitions in vmparam.h to change, and the allocation of > some additional PDP-level page table pages in create_pagetables(). > In contrast, the changes to break the original 2GB KVM barrier > involved touching a number of different places in the kernel. Alan, since some people (e.g. Sean Bruno) were hitting this, do you think you could provide a patch to allow people to test this? Sean in Particular seemed keen to try go to 1TB ram in a machine he had access to. Julian > > Alan > > > >
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