Re: perspectives on 32 bit vs 64 bit
Helge Hafting wrote:
The limit is 4GB.
No, wrong. The limit is 64 GiB physical RAM, period. PAE has been part
of the ISA for pratically forever now, so it's silly to say it's
anything else.
However, practically, Linux can only use 16 GiB physical RAM without
special patches, because of how the virtual memory is split by default
(1GiB/3GiB). Windows is subject to the same limitation.
Fully getting around this requires a 4G/4G patch, which is a terrible
performance penalty because it requires a full TLB flush on every
context switch.
> (Well,
you can theoretically address more, but you definitely don't want
to do the work necessary to do that.
No, you cannot. One process cannot address more than 4 GiB virtual
memory at a time, period. The only way to give more physical addressing
is via mapping techniques, which is what Windows does with AWE. In
eitehr case, you only have 4 GiB of virtual memory available at once.
But the gain of that was questionable when it was first implemented in
Windows, and is even more so now.
See: http://blogs.msdn.com/oldnewthing/archive/2004/08/18/216492.aspx
Adam
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