Re: Q: PREFETCH_STRIDE/16

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

>>>>> On Thu, 23 May 2002 09:34:16 -0700 (PDT), "David S. Miller" <davem@redhat.com> said:

  DaveM> I mentioned this 1 or 2 days ago in the TLB thread with
  DaveM> Linus, you pessimistically maintain a tiny bitmap per
  DaveM> mm_struct which keeps track of where mappings actually are.
  DaveM> You use some hash function on the virtual address to
  DaveM> determine the bit.  You clear it when the mm_struct is new,
  DaveM> and you just set bits when mappings are installed.  Very
  DaveM> simple.

  DaveM> Then all of these "walk all valid page tables" loops that
  DaveM> scan entire mostly empty pages of pgd/pmd/pte entries for no
  DaveM> reason can just check the bitmap instead.

  DaveM> Most of the exit overhead is in clear_page_tables walking
  DaveM> over entire pages.  It effectively flushes the cache unless
  DaveM> all you are doing is fork/exit/fork/exit

Sounds like something worth experimenting with.  I doubt you could
really avoid (effectively) flushing the caches, but even if there are
just a few zero bits in the bitmap at the time of the tear-down, a
fair amount of time could be saved.

	--david