Subject: Re: [PATCH] iptables: lock free counters

From: Eric Dumazet <[email-masked]>
Date: Fri, 20 Feb 2009 00:46:17 +0100

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 <[email-masked]>

Acked-by: Eric Dumazet <[email-masked]>

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 :)

> 
> ---
> Added missing preempt_enable.  Patch against nf-next-2.6 git tree.
> 
>  include/linux/netfilter/x_tables.h |    6 +
>  net/ipv4/netfilter/arp_tables.c    |  115 +++++++++++++++++++++++++++--------
>  net/ipv4/netfilter/ip_tables.c     |  120 ++++++++++++++++++++++++++-----------
>  net/ipv6/netfilter/ip6_tables.c    |  119 +++++++++++++++++++++++++-----------
>  net/netfilter/x_tables.c           |   26 ++++++--
>  5 files changed, 284 insertions(+), 102 deletions(-)
>