Re: [rfc] Near-constant time directory index for Ext2

[Daniel Phillips]

Linus Torvalds wrote:
> 
> On Tue, 20 Feb 2001, Daniel Phillips wrote:
> >
> > You mean full_name_hash?  I will un-static it and try it.  I should have
> > some statistics tomorrow.  I have a couple of simple metrics for
> > measuring the effectiveness of the hash function: the uniformity of
> > the hash space splitting (which in turn affects the average fullness
> > of directory leaves) and speed.
> 
> I was more thinking about just using "dentry->d_name->hash" directly, and
> not worrying about how that hash was computed. Yes, for ext2 it will have
> the same value as "full_name_hash" - the difference really being that
> d_hash has already been precomputed for you anyway.
> 
> > Let the hash races begin.
> 
> Note that dentry->d_name->hash is really quick (no extra computation), but
> I'm not claiming that it has anything like a CRC quality. And it's
> probably a bad idea to use it, because in theory at least the VFS layer
> might decide to switch the hash function around. I'm more interested in
> hearing whether it's a good hash, and maybe we could improve the VFS hash
> enough that there's no reason to use anything else..

In the first heat of hash races - creating 20,000 files in one directory
- dentry::hash lost out to my original hack::dx_hash, causing a high
percentage of leaf blocks to remain exactly half full and slowing down
the whole thing by about 5%.  (This was under uml - I haven't tried it
native yet but I expect the results to be similar.)

	  Contender			Result
	  =========			======
	dentry::hash		Average fullness = 2352 (57%)
	hack::dx_hash		Average fullness = 2758 (67%)

This suggests that dentry::hash is producing distinctly non-dispersed
results and needs to be subjected to further scrutiny.  I'll run the
next heat of hash races tomorrow, probably with R5, and CRC32 too if I
have time.

--
Daniel