Re: Why *not* rmap, anyway?

[Rik van Riel <riel@conectiva.com.br>]

On Wed, 24 Apr 2002, Christian Smith wrote:
> On Tue, 23 Apr 2002, Rik van Riel wrote:
> >On Tue, 23 Apr 2002, Christian Smith wrote:
> >
> >> The question becomes, how much work would it be to rip out the Linux MM
> >> piece-meal, and replace it with an implementation of UVM?
> >
> >I doubt we want the Mach pmap layer.
>
> Why not? It'd surely make porting to new architecures easier (not that
> I've tried it either way, mind)

You really need to read the pmap code and interface instead
of repeating the statements made by other people. Have you
ever taken a close look at the overhead implicit in the pmap
layer ?


> interface. Pmap can hide the differences between forward mapping page
> table, TLB or inverted page table lookups, can do SMP TLB shootdown
> transparently. If not the Mach pmap layer, then surely another pmap-like
> layer would be beneficial.

Then how about the Linux pmap layer ?

The datastructure is a radix tree, which happens to map 1 to 1
with the MMU on most architectures. On architectures that don't
have forward page tables Linux fills in the hardware's translation
tables with data from those radix trees.


> It can handle sparse address space management without the hackery of
> n-level page tables, where a couple of years ago, 3 levels was enough for
> anyone, but now we're not so sure.
>
> The n-level page table doesn't fit in with a high level, platform
> independant MM, and doesn't easily work for all classes of low level MMU.
> It doesn't really scale up or down.

Do you have any arguments or are you just repeating what you
read somewhere else ?

Just think about it for a second ... the radix tree structure
of page tables are as good a datastructure as any other.

The mythical "sparse mappings" seem to be very rare in real
life and I'm not convinced they are a reason to change all of
our VM.


regards,

Rik
-- 
	http://www.linuxsymposium.org/2002/
"You're one of those condescending OLS attendants"
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http://www.surriel.com/		http://distro.conectiva.com/

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