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

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Date: Sat, 24 Feb 2024 22:20:48 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: David Laight <David.Laight@...lab.com>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Thomas Graf <tgraf@...g.ch>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, 
	"maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>, "rcu@...r.kernel.org" <rcu@...r.kernel.org>
Subject: Re: [PATCH 0/1] Rosebush, a new hash table

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.