netdev - Re: [PATCH] iptables: lock free counters

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Date:	Fri, 20 Feb 2009 10:37:45 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Rick Jones <rick.jones2@...com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] iptables: lock free counters

Eric Dumazet wrote:
> Stephen Hemminger a écrit :
>> The reader/writer lock in ip_tables is acquired in the critical path of
>> processing packets and is one of the reasons just loading iptables can cause
>> a 20% performance loss. The rwlock serves two functions:
>>
>> 1) it prevents changes to table state (xt_replace) while table is in use.
>>    This is now handled by doing rcu on the xt_table. When table is
>>    replaced, the new table(s) are put in and the old one table(s) are freed
>>    after RCU period.
>>
>> 2) it provides synchronization when accesing the counter values.
>>    This is now handled by swapping in new table_info entries for each cpu
>>    then summing the old values, and putting the result back onto one
>>    cpu.  On a busy system it may cause sampling to occur at different
>>    times on each cpu, but no packet/byte counts are lost in the process.
>>
>> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> 
> 
> Acked-by: Eric Dumazet <dada1@...mosbay.com>
> 
> Sucessfully tested on my dual quad core machine too, but iptables only (no ipv6 here)
> 
> BTW, my new "tbench 8" result is 2450 MB/s, (it was 2150 MB/s not so long ago)
> 
> Thanks Stephen, thats very cool stuff, yet another rwlock out of kernel :)

Applied, thanks everyone. I've also addes Eric's tbench results
to the changelog.
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