netdev - Re: [net-next PATCH V2 8/9] net: frag queue locking per hash bucket

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Date:	Fri, 30 Nov 2012 13:55:32 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Thomas Graf <tgraf@...g.ch>, Cong Wang <amwang@...hat.com>,
	Patrick McHardy <kaber@...sh.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [net-next PATCH V2 8/9] net: frag queue locking per hash bucket

On Thu, 2012-11-29 at 09:08 -0800, Eric Dumazet wrote:

> So we still have this huge contention on the reader-writer lock cache
> line...
> I would just remove it. (And remove hash rebuild, or make it RCU
> compatible )

I vote for removing the rebuild feature.  It really doesn't make sense
to protect a 256Kb memory (using) hash, with very short lived elements,
against hash collision attacks, every 10 minutes. (Especially as we saw
a 7 GBit/s performance improvement)

Making the hash RCU "compatible" is not going to work, because the
elements are too short lived, thus we cannot free them fast enough
(call_rcu stuff). (I actually did the RCU implementation, to realize
this...)

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