Re: MIPS addressing limits, Was: Re: CVS Update@-mips.org: linux

[Jun Sun <jsun@mvista.com>]

On Wed, Oct 15, 2003 at 04:59:48PM +0200, Ralf Baechle wrote:
> On Wed, Oct 15, 2003 at 04:23:06PM +0200, Maciej W. Rozycki wrote:
> 
> > > Still want more?  A 3 level tree would then cover 128TB of virtual
> > > address space already exceedin the hardware limits of all processors but
> > > the R8000.
> > 
> >  Well, the MIPS64 ISA spec allows up to 8EB of user memory to be supported
> > by an implementation, IIRC; probably nothing supports that much yet,
> > though. ;-)  BTW, is an R8000 spec available online anywhere?
> 
> There used to be a few papers published by SGI online and various other
> bits of information I found through google.
> 
> (I happen to have a paper copy of the R8000 manual but since the responsible
> people still haven't informed me if I can legally use it, this book is
> closed and will stay closed until this happens - if ever ...  Pitty, I
> still receive mails from various R8000 users ...)
> 
> > > 64k pagesize stretches the limits even further.   Here a two level
> > > pagetable tree would cover 4TB, 3-level could cover 32PB exceeding
> > > the capacity of every MIPS processor ever made - and probably sufficient
> > > for the coming decade :-)
> > 
> >  Further increasing of the page size should result in better performance
> > due to fewer TLB misses and reduce the memory footprint of page tables,
> > but the drawback is more memory is wasted for maps.  Whether the end
> > result is a gain or a loss depends on the actual application of a system,
> > so I guess we should either leave the size configurable (with a sane
> > default for those who might have troubles judging what would suit them
> > best) or only decide on a given size after lots of benchmarking.
> 
> Unless somebody yells I almost feel like ditching 3-level pagetable
> support; 2-levels with a decent pagesize should suffice for a few years
> to come ...
> 

Isn't ia64 still using 3-level page tables?  Any performance data we
can infer from theirs?

I feel a little uneasy about ditching 3-level pagetable altogether.
Leaving all the parameters configurable, including the possiblity of
nullifying the second level and changing page size, seems to be a more 
comforting thought.

Jun