Re: RFC: two-level page tables for 16KB pages on LP64 kernels

[Michael <macallan1888%gmail.com@localhost>]

Hello,

On Sun, 28 Aug 2011 23:37:22 +0200
Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:

> On Sun, Aug 28, 2011 at 02:33:51PM -0700, Matt Thomas wrote:
> > 
> > On Aug 28, 2011, at 2:32 PM, Manuel Bouyer wrote:
> > 
> > > On Fri, Aug 26, 2011 at 03:35:13PM -0700, Matt Thomas wrote:
> > >> 
> > >> Normally, MIPS uses a 3 level page table for a 64-bit kernel which gives 
> > >> 1TB (40 [12+10+9+9] bits) of user address space when using 4KB pages.  A 
> > >> 3 level page table using 16KB pages can address 256TB (48 [14+12+11+11] 
> > >> bits) while a 2 level page table using 16KB pages can address 128GB (37 
> > >> [14+12+11] bits) of address space.
> > >> 
> > >> Thinking about it, 128GB seems to more than enough on the GDIUM or other 
> > >> Loongson2 base platforms but it is significantly less the 16TB that it 
> > >> could use.  I have to think the smaller (but adequate) address space 
> > >> using a simple 2 level page table might result in a small performance 
> > >> boost.
> > >> 
> > >> I figure this could be a config file option.  Maybe even defining 
> > >> VM_MAX_USER_ADDRESS in an option file and letting the kernel figure out 
> > >> how many levels are needed to accomodate that maximum.
> > > 
> > > 
> > > How many instructions will it save in the trap handler ?
> > > With 16k pages we should already have less TLB miss; I'm not sure the
> > > performance gain justifies the maintenance cost of having another TLB
> > > handler ...
> > > but if you have patches to test, I'd be happy to try and benchmark on
> > > my fuloong.
> > 
> > It will also save 1 16KB page per process (pmap).
> 
> That's little gain on a system that can take 1Gb of RAM, but maybe you
> have in mind some embeded systems that could also benefit from this ?

This might be useful for some sgimips boxes - Indy for example can't take more 
than 256MB RAM, older machines have lower limits and RAM for (slightly) newer 
machines is made of unobtainium.
( and then there's the whole mess with sgimips assuming we can access 
everything through KSEGx )

have fun
Michael