limits to memory on amd64

[Alan Cox]

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 at 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