Re: [patch] arca-vm-2.2.5

Chuck Lever (cel@monkey.org)
Tue, 6 Apr 1999 11:38:52 -0400 (EDT)

On Tue, 6 Apr 1999, Stephen C. Tweedie wrote:
> On Mon, 5 Apr 1999 17:31:43 -0400 (EDT), Chuck Lever <cel@monkey.org>
> said:
>
> > hmmm. wouldn't you think that hashing with the low order bits in the
> > offset would cause two different offsets against the same page to result
> > in the hash function generating different output?
>
> We always, always use page-aligned lookups for the page cache.
> (Actually there is one exception: certain obsolete a.out binaries, which
> are demand paged with the pages beginning at offset 1K into the binary.
> We don't support cache coherency for those and we don't support them at
> all on filesystems with a >1k block size. It doesn't impact on the hash
> issue.)

i guess i'm confused then. what good does this change do:

2.2.5 pagemap.h:
#define i (((unsigned long) inode)/(sizeof(struct inode) &
~ (sizeof(struct inode) - 1)))
#define o (offset >> PAGE_SHIFT)
#define s(x) ((x)+((x)>>PAGE_HASH_BITS))
return s(i+o) & (PAGE_HASH_SIZE-1);

2.2.5-arca pagemap.h:
return s(i+o+offset) & (PAGE_HASH_SIZE-1);
^^^^^^^

btw, do you know if there is a special reason why PAGE_HASH_BITS is 11?

- Chuck Lever

--
corporate:	<chuckl@netscape.com>
personal:	<chucklever@netscape.net> or <cel@monkey.org>


The Linux Scalability project:
	http://www.citi.umich.edu/projects/citi-netscape/




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