linux-kernel - Re: [PATCH v5 2/2] netfilter: save the hash of the tuple in the original direction for latter use

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Date:	Tue, 21 Sep 2010 08:02:43 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] netfilter: save the hash of the tuple in the
 original direction for latter use

On Tue, Sep 21, 2010 at 1:08 AM, Patrick McHardy <kaber@...sh.net> wrote:
>
> Sure we can, dropping unconfirmed conntracks is a rare exception,
> not a common case. Even under DoS we usually drop *unassured*
> conntracks, which have already enterered the hash. If we're unable
> to do that, we won't even allocate a new conntrack.
>

Even so, saving the hash of the reply tuple isn't a good idea.

If NAT is turned on, the current code is:

mangle the reply tuple -> compute the hash of the reply tuple ->
insert into the conntrack hash table.

the new code is

compute the hash of the reply tuple -> mangle the reply tuple ->
recompute the hash of the reply tuple -> insert into the conntrack
hash table.

As you see, the hash computing is done twice, and we use more CPU
cycles than before.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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