Date: Tue, 09 Nov 2010 09:20:41 -0800 From: Julian Elischer <julian@freebsd.org> To: Bakul Shah <bakul@bitblocks.com> Cc: alc@freebsd.org, FreeBSD Current <current@freebsd.org> Subject: Re: limits to memory on amd64 Message-ID: <4CD982E9.70500@freebsd.org> In-Reply-To: <20101109170453.942D95B89@mail.bitblocks.com> References: <4CD97A9A.8000007@freebsd.org> <20101109170453.942D95B89@mail.bitblocks.com>
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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? > > From Wikipedia: > > Larger physical address space: The original > implementation of the AMD64 architecture implemented > 40-bit physical addresses and so could address up to 1 TB > (2^40 bytes) of RAM. Current implementations of the AMD64 > architecture (starting from AMD 10h microarchitecture) > extend this to 48-bit physical addresses and therefore > can address up to 256 TB of RAM. The architecture permits > extending this to 52 bits in the future (limited by the > page table entry format); this would allow addressing of > up to 4 PB of RAM. >
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