netdev - Re: Performance regression from routing cache removal?

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Date:	Wed, 05 Jun 2013 11:13:17 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shawn Bohrer <sbohrer@...advisors.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: Performance regression from routing cache removal?

On Wed, 2013-06-05 at 12:57 -0500, Shawn Bohrer wrote:
> I've got a performance regression that I've been trying to track down
> for the last couple of days.  The last known good kernel was 3.4 and
> now I'm testing 3.10 so there has been a lot of changes in between.
> The workload I'm testing has a single machine receiving UDP multicast
> packets across approximately 350 multicast groups.  We've got one
> socket per multicast address receiving the data.  The traffic is small
> packets and tends to be very bursty.  With the 3.10 kernel I'm seeing
> occasional spikes in one-way latency that are in the 100-500
> millisecond range, and regularly in the 10 millisecond range.  I've
> started a git bisect which has narrowed me down to:
> 
> bad f5b0a8743601a4477419171f5046bd07d1c080a0
> good fa0afcd10951afad2022dda09777d2bf70cdab3d
> 
> The remaining commits are all part of the routing cache removal so
> I've stopped my bisection for now.  I can bisect further if anyone
> thinks it will be valuable.

> 
> Digging through the git history the ip_check_mc_rcu appears to be the
> result of the routing cache removal. 

Yes, ip_check_mc_rcu() does a linear scan of your ~350 groups :

for_each_pmc_rcu(in_dev, im) {
    if (im->multiaddr == mc_addr)
          break;
}



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