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From: David Mosberger <davidm@napali.hpl.hp.com>
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Date: Fri, 2 Aug 2002 20:19:59 -0700
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Gerrit Huizenga <gh@us.ibm.com>, Hubertus Franke <frankeh@watson.ibm.com>,
       <Martin.Bligh@us.ibm.com>, <wli@holomorpy.com>,
       Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: large page patch (fwd) (fwd) 
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>>>>> On Fri, 2 Aug 2002 12:34:08 -0700 (PDT), Linus Torvalds <torvalds@transmeta.com> said:

  Linus> We may well expand the FS layer to bigger pages, but "bigger"
  Linus> is almost certainly not going to include things like 256MB
  Linus> pages - if for no other reason than the fact that memory
  Linus> fragmentation really means that the limit on page sizes in
  Linus> practice is somewhere around 128kB for any reasonable usage
  Linus> patterns even with gigabytes of RAM.

  Linus> And _maybe_ we might get to the single-digit megabytes. I
  Linus> doubt it, simply because even with a good buddy allocator and
  Linus> a memory manager that actively frees pages to get large
  Linus> contiguous chunks of RAM, it's basically impossible to have
  Linus> something that can reliably give you that big chunks without
  Linus> making normal performance go totally down the toiled.

The Rice people avoided some of the fragmentation problems by
pro-actively allocating a max-order physical page, even when only a
(small) virtual page was being mapped.  This should work very well as
long as the total memory usage (including memory lost due to internal
fragmentation of max-order physical pages) doesn't exceed available
memory.  That's not a condition which will hold for every system in
the world, but I suspect it is true for lots of systems for large
periods of time.  And since superpages quickly become
counter-productive in tight-memory situations anyhow, this seems like
a very reasonable approach.

	--david
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