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

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Message-ID: <ZdqO3G6Fb4wYhVEj@gondor.apana.org.au>
Date: Sun, 25 Feb 2024 08:50:36 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: David Laight <David.Laight@...lab.com>
Cc: "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 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@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt