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

[Ralf Baechle <ralf@linux-mips.org>]

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 ...

  Ralf