netdev - Re: [RFT 4/4] netfilter: Get rid of central rwlock in tcp conntracking

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Date:	Thu, 19 Feb 2009 00:23:45 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	David Miller <davem@...emloft.net>
CC:	shemminger@...tta.com, rick.jones2@...com, dada1@...mosbay.com,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [RFT 4/4] netfilter: Get rid of central rwlock in tcp conntracking

David Miller wrote:
> From: Patrick McHardy <kaber@...sh.net>
> Date: Wed, 18 Feb 2009 10:56:45 +0100
> 
>> Eric already posted a patch to use an array of locks, which is
>> a better approach IMO since it keeps the size of the conntrack
>> entries down.
> 
> Just as a side note, we generally frown upon the
> hash-array-of-spinlocks approach to scalability.
> 
> If you need proof that in the long term it's suboptimal, note that:
> 
> 1) this is Solaris's approach to locking scalability :-)

:)

> 2) every such case in the kernel eventually gets transformed into
>    RCU, a tree/trie based scheme, or some combination of the two
> 
> So maybe for now it's ok, but keep in mind that eventually
> this is certain to change. :)

This case might be different in that a normal firewall use case
probably doesn't have more than 16 cpus, even than would be quite
a lot. So for bigger machines this is probably more about keeping
the "non-use" costs low.

I'll keep it in mind though and I'm interested in seeing how it
turns out in the long term :)
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