Re: large page patch (fwd) (fwd)

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Sat, 3 Aug 2002 12:43:47 -0700 (PDT), Linus Torvalds <torvalds@transmeta.com> said:

  >> You don't need separate system calls for that: with a transparent
  >> superpage framework and a privileged & reserved giant-page pool,
  >> it's trivial to set up things such that your favorite data base
  >> will always be able to get the giant pages (and hence the giant
  >> TLB mappings) it wants.  The only thing you lose in the
  >> transparent case is control over _which_ pages need to use the
  >> pinned giant pages.  I can certainly imagine cases where this
  >> would be an issue, but I kind of doubt it would be an issue for
  >> databases.

  Linus> That's _probably_ true. There aren't that many allocations
  Linus> that ask for megabytes of consecutive memory that wouldn't
  Linus> want to do it. However, there might certainly be non-critical
  Linus> maintenance programs (with the same privileges as the
  Linus> database program proper) that _do_ do large allocations, and
  Linus> that we don't want to give large pages to.

  Linus> Guessing is always bad, especially since the application
  Linus> certainly does know what it wants.

Yes, but that applies even to a transparent superpage scheme: in those
instances where an application knows what page size is optimal, it's
better if the application can express that (saves time
promoting/demoting pages needlessly).  It's not unlike madvise() or
the readahead() syscall: use reasonable policies for the ordinary
apps, and provide the means to let the smart apps tell the kernel
exactly what they need.

	--david