netdev - Re: [PATCH net-next] ipv6: reduce percpu needs for icmpv6msg mibs

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Date:	Mon, 14 Nov 2011 08:12:53 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] ipv6: reduce percpu needs for icmpv6msg mibs

On Sun, 13 Nov 2011 12:24:04 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Reading /proc/net/snmp6 on a machine with a lot of cpus is very
> expensive (can be ~88000 us).
> 
> This is because ICMPV6MSG MIB uses 4096 bytes per cpu, and folding
> values for all possible cpus can read 16 Mbytes of memory (32MBytes on
> non x86 arches)
> 
> ICMP messages are not considered as fast path on a typical server, and
> eventually few cpus handle them anyway. We can afford an atomic
> operation instead of using percpu data.
> 
> This saves 4096 bytes per cpu and per network namespace.

Probably should do this for ICMP for IPv4 and all the other
non hot counters. The whole per-cpu SNMP MIB stuff for all counters
is massive overkill.
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