Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sun, 16 Sep 2007, Jörn Engel wrote:
> 
> I have been toying with the idea of having seperate caches for pinned
> and movable dentries.  Downside of such a patch would be the number of
> memcpy() operations when moving dentries from one cache to the other.

Totally inappropriate.

I bet 99% of all "dentry_lookup()" calls involve turning the last dentry 
from having a count of zero ("movable") to having a count of 1 ("pinned").

So such an approach would fundamentally be broken. It would slow down all 
normal dentry lookups, since the *common* case for leaf dentries is that 
they have a zero count.

So it's much better to do it on a "directory/file" basis, on the 
assumption that files are *mostly* movable (or just freeable). The fact 
that they aren't always (ie while kept open etc), is likely statistically 
not all that important.

			Linus
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