netdev - Re: [v1 PATCH 1/14] rhashtable: Remove shift from bucket

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Message-ID: <20150317112203.GG11089@casper.infradead.org>
Date:	Tue, 17 Mar 2015 11:22:03 +0000
From:	"tgraf@...g.ch" <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Laight <David.Laight@...LAB.COM>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [v1 PATCH 1/14] rhashtable: Remove shift from bucket_table

On 03/17/15 at 10:00pm, Herbert Xu wrote:
> On Tue, Mar 17, 2015 at 10:56:57AM +0000, tgraf@...g.ch wrote:
> >
> > Given the discussions, the grow decision will likely change to
> > a max bucket length limit anyway.
> 
> Actually no.  In my pathces the chain length is only used to
> force an immediate rehash.  Growing is still based on the number
> of elements.
> 
> The reason is that the maximum (not average) chain length actually
> grows with the hash table size, even at 75% utilisation.
> 
> So unless we want ever decreasing utilisation as the table grows,
> we have to cope with a few chains with many elements.  The limit
> I'm currently using is 16 which shouldn't be hit even if you had
> a 2^32 table (which you currently cannot).

Not sure I understand the downside of a bucket length based grow
decision with optional forced rehashing plus synchroneous table
realloc if we hit a 2nd watermark as you proposed earlier. Shouldn't
we consider deterministic lookup and insert behaviour more important
than overall table utilization? Given the rehashing, the grow decision
should not be attackable.
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