From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from burns.conectiva (burns.conectiva [10.0.0.4]) by perninha.conectiva.com.br (Postfix) with SMTP id BDAE439A68 for ; Wed, 24 Apr 2002 11:20:07 -0300 (EST) Date: Wed, 24 Apr 2002 11:20:05 -0300 (BRT) From: Rik van Riel Subject: Re: Why *not* rmap, anyway? In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Christian Smith Cc: Joseph A Knapka , "Martin J. Bligh" , linux-mm@kvack.org List-ID: 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" "Here's a nickle kid. Go buy yourself a real t-shirt" http://www.surriel.com/ http://distro.conectiva.com/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/