[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20150317121847.GK17829@casper.infradead.org>
Date: Tue, 17 Mar 2015 12:18:47 +0000
From: "'tgraf@...g.ch'" <tgraf@...g.ch>
To: David Laight <David.Laight@...LAB.COM>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
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 12:13pm, David Laight wrote:
> From: Thomas Graf
> > Sent: 17 March 2015 11:58
> ...
> > > 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.
>
> I think you are seriously overestimating the 'efficiency' of the hash function.
> And not doing the 'birthday paradox' maths at all.
>
> The only way you'll get a 'hash' that good is if you can pick the input
> value in order to generate a perfect hash.
> However you aren't going to manage that for inbound TCP connections since
> none of the inputs to the hash can be chosen by the listening system
> (unless IPv6 has something than can help you).
>
> You may have to live with a few % of the items being on long chains.
> Maybe count the number of items on chains longer than (say) 4 and
> rehash or increase the table size if this exceeds a few % of the
> table size.
> (Or count the number of items that are further than 4 from the start
> of the hash chain.)
Did you read my email all the way? You basically omitted to quote and
then rephrased the 2nd half of my email.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists