> I've just had someone bashing Linux because of the 4GB max physical limits
> for Intel architectures. I've told him to fuck off and buy an 64bit Alpha,
> but he mentioned that BSDI can use all 64GB on a Xeon's 36bit addressing
> range. Why is Linux limited to 4GB and BSDI can address 64GB ona Xeon?
May I risk my explanation ? I will do. ;)
Unlike Linux, other UNIX kernels do not seem to map the whole physical
memory from the kernel, but only on need. Basically, when they want to
access user private memory, they map the correspoding memory area into the
kernel virtual addressing space. With such a design, using more than 4 GB
of memory (or the whole 4 GB space) on a 32 bit system is probably far
easier to implement than on Linux.
The cost of such a design is far more TLB invalidations than on linux, and
probably some extra cost on some architecture, perhaps on those that use
virtually mapped cache. Gurus may add some other good reasons that
demonstrate that Linux choice is better.
Anyway, Virtual Memory has been invented to provide application a virtual
memory space far larger than the available physical space.
The 36 bit addressing scheme on IA32 is an horrible hack, because Intel
has been unable to supply IA64 in time. Encouraging support of this hacked
architecture is technically stupid, in my opinion.
Gérard.
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