Vic wrote:
>> A 32-bit OS can address about 3.5 GB of RAM.
>
> That's Windows. Linux has far superior memory handling - as long as you
> use it.
>
>> If you need anything more than this the host OS will need to be 64 bit as
>> it needs to be able to access all the RAM in use
>
> The 32-bit hugemem kernels from RHEL will address 64GB. For RHEL3, you got
> a 4GB limit per process - I doubt that's changed, but I don't actually
> know for sure.
I doubt it can change. I assume that those kernels make use of the
Physical Address Extensions to access the larger memory pool, and the
way that works, very simply, is that each 32 bit process gets it's own
32bit (4Gb) address space, but the kernel, with hardware help, can map
that into a region of a 64 bit address space.
I previously compared it to the old DOS extender trick of mapping
regions above the 1Mb level into "buffers" below the 1Mb area, so that
"real mode" programs could still access more than 1Mb of data, despite
only having 1Mb of address space.
PAE is a bit more sophisticated, and less hacky.
I've no idea about the performance impact compared to a "flat" 64 bit
address space, but I suspect it's not too bad given the hardware
assistance, and the fact it's not a time consuming mode change for the CPU.
For anyone sad enough (like me!) to be interested:
http://en.wikipedia.org/wiki/Physical_Address_Extension
Chris
