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From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: [Lse-tech] Re: 10.31 second kernel compile
Date: Thu, 14 Mar 2002 19:05:50 +0000 (UTC)
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In article <87wuwfxp25.fsf@fadata.bg>,
Momchil Velikov  <velco@fadata.bg> wrote:
>
>Out of curiousity, why there's a need to update the linux page tables ?
>Doesn't pte/pmd/pgd family functions provide enough abstraction in
>order to maintain _only_ the hashed page table ?

No.  The IBM hashed page tables are not page tables at all, they are
really just a bigger 16-way set-associative in-memory TLB. 

You can't actually sanely keep track of VM layout in them.

Those POWER4 machines are wonderful things, but they have a few quirks:

 - it's so expensive that anybody who is slightly price-conscious gets a
   farm of PC's instead. Oh, well.

 - the CPU module alone is something like .5 kilowatts (translation:
   don't expect it in a nice desktop factor, even if you could afford
   it). 

 - IBM nomenclature really is broken. They call disks DASD devices, and
   they call their hash table a page table, and they just confuse
   themselves and everybody else for no good reason.  They number bits
   the wrong way around, for example (and big-endian bitordering really
   _is_ clearly inferior to little-endian, unlike byte-ordering.  Watch
   the _same_ bits in the _same_ register change name in the 32 vs
   64-bit architecture manuals, and puke)

But with all their faults, they do have this really studly setup with 8
big, fast CPU's on a single module. A few of those modules and you get
some ass-kick performance numbers. As you can see.

		Linus
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