Re: [PATCH 0/1] Rosebush, a new hash table

[Kent Overstreet]

On Sun, Feb 25, 2024 at 08:50:36AM +0800, Herbert Xu wrote:
> On Sat, Feb 24, 2024 at 10:10:27PM +0000, David Laight wrote:
> >
> > > Normally an rhashtable gets resized when it reaches 75% capacity
> > > so the average chain length should always be one.
> > 
> > The average length of non-empty hash chains is more interesting.
> > You don't usually search for items in empty chains.
> > The only way you'll get all the chains of length one is if you've
> > carefully picked the data so that it hashed that way.
> 
> Sure.  But given the 75% capacity, you'd need a really bad hash
> function to get an *average* (not worst-case) chain length of
> 10.
> 
> > I remember playing around with the elf symbol table for a browser
> > and all its shared libraries.
> > While the hash function is pretty trivial, it really didn't matter
> > whether you divided 2^n, 2^n-1 or 'the prime below 2^n' some hash
> > chains were always long.
> 
> Even in the unlikely event of bad luck and everything bunches up
> together, we change theh hash function (through hash_rnd) every
> time we resize so you would expect things to even out after the
> resize event.
> 
> A rehash is also automatically triggered if the worst-case chain
> length exceeds 16.

16!? that's crap, use a decent hash function and 3-5 should be your
worst upper bound.