Yes, the mapping/unmapping can be expensive, depending upon
the underlying hardware architecture. The code to implement it adds
complexity to kernel services and device drivers. It also introduces
another set of potential deadlocks and resource mismanagements.
There is a question of strategy in managing the kernel's
evolution in a changing world. The limitations of the kernel's
physical memory map vanish on a properly implemented 64-bit
architecture. *If* you believe that 64-bit machines are now (or are
soon to be) more cost-effective than 32-bit ones, and if the memory
problem is one that affects only top-end machines, then is it worth
putting a lot of effort into fixing top-end performance on 32-bit
machines, when the same effort can yield greater results on 64-bit
machines?
Craig Milo Rogers
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