How FreeBSD manage more than 4GB on 32 bits architecture

[Pierre-Yves Péneau]

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On 04/09/2015 09:07 PM, John Levine wrote:
>> So, I've the required hardware I think. My problem is to find out
>> how FreeBSD can use this mechanisms. But I'm realizing that it
>> may be not a kernel issue, because it's an hardware functionality
>> which should works with Linux too. Right ?
> 
> Right.  I don't think I've ever seen a machine with a 32 bit 
> architecture that could address more than 36 bits of real memory.
> If you need more than that, you use a 64 bit architecture.

But it exists :-)  We have a simulate environment, and a real
prototype which have 24 clusters (i.e. 96 CPUs) and 96GB of physical
memory. All of this with 32 RISC processors.

> As far as I know, 32 and 64 bit versions of FreeBSD use the same 
> virtual memory manager, and there's no inherent reason you
> couldn't manage large physical memories with 32 bit code.  But I
> can tell you that you will be sorry if you try.  If the physical
> memory is much bigger than a program's logical address space, you
> have to do some sort of tricks for programs to access all of the
> memory.  If the memory is only a few times larger than the address
> space, say 36 bits, it's OK to limit programs to 32 bits and hope
> you have enough of them to keep the memory busy.  Or you use tricks
> like large shared files that are mapped in and out of the program's
> address space, typically with a blizzard of TLB misses every time.
> 
> But if you have 40 bits, you really want something that can address
> 40 bits which in today's world means a 64 bit architecture.  Take a
> look at ARMv8 which handles 48 bit physical addresses and is
> supposed to have low power implementations for smartphones and
> other power-constrained environments.
> 
> R's, John
> 
> 
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> 

Thank you for your advices. I agree with you, but it's an internship,
I can't say "Please, would you like to change all your CPUs for me ?"
:-)   Moreover, the main challenge is to deal with huge memory with 32
bits processors (only RISC), I think it's really interesting.

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