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

[Herbert Xu]

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.

Cheers,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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