Re: Why preallocate pmd in x86 32-bit PAE?

[H. Peter Anvin]

William Lee Irwin III wrote:
> There may be bigger fish to fry in terms of per-process overhead, if
> you're trying to cut that down. The trouble with trying to address
> some of those is that there is mutual antagonism between compactness
> and expansibility in the process address space layout, so you'll end
> up instantiating a lot more than you want barring some sort of provision
> for a compact address space layout. Pagetable sharing is a far more
> powerful resource scalability method, though it also needs cooperation
> in user address space layout to reap its gains.
>
> There are other overheads, of course, though they're more typically
> per-something besides processes.

I think Jeremy's question was due to trying to reduce the 32/64-bit 
differences.  Performance-wise, it might add a small amount to user 
setup time (a typical 32-bit process will need all four, for the main 
binary, libraries, stack and kernel, respectively) but it is probably 
not significant (although I'd like to see numbers just in case).

	-hpa

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