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SubjectRe: bad lmbench numbers for mmap
   Date: Sat, 17 Apr 1999 03:11:45 +0200
From: Jamie Lokier <lkd@tantalophile.demon.co.uk>

Speaking of which. In principle, freeing page tables should make
munmap() *faster*, because then you don't have to clear the page
tables.

I note 2.2.6-pre3 isn't doing that.

Yes do have to clear them, for two reasons:

1) So that the hardware MMU doesn't see valid entries after the
tlb flush. (or in the case of clever software TLB miss strategies
like that implemeted in the Alpha PAL code or in our sparc64 port,
making sure the virtual page table mappings go away properly too)

2) For the sake of the page table caching we do on all ports now.

For #2, if a page table chunk is cached, it works so efficiently
because it knows the chunks have been cleared out by the callers at
some point before being free'd.

Later,
David S. Miller
davem@redhat.com

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