limits to memory on amd64

[Julian Elischer]

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 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,
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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