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

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Message-ID: <20150317115749.GJ17829@casper.infradead.org>
Date:	Tue, 17 Mar 2015 11:57:49 +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 10:27pm, Herbert Xu wrote:
> On Tue, Mar 17, 2015 at 11:22:03AM +0000, tgraf@...g.ch wrote:
> >
> > 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.
> 
> Do you really want to double the table size when 0.1% of the buckets
> have a chain length > 4 but still < 16?

If we constantly hit that bucket because we are handling just a few
TCP flows it would be worth to double the size & rehash to avoid the
additional cache misses of the linked list.

Although a limit of 4 would be too high. Ideally we should resize and
rehash when we add the 2nd entry to a bucket to stay < 100% utilization.
It seems likely though that we'll always have a bucket with >=2
entries so we would end up constantly doubling and rehashing. This is
the only thing that speaks for a table wide nelems counters in my
opinion.

Another option would be to continue resizing & rehashing as long as we
have at least one bucket with >= 4 entries but allow a table size
dependant limit of (length > 1 && length < 4) buckets. This may be
overengineered again though ;-)
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