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
