netdev - Re: High perf top ip_idents_reserve doing netperf UDP

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Message-ID: <1409757426.26422.41.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 03 Sep 2014 08:17:06 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>
Subject: Re: High perf top ip_idents_reserve doing netperf UDP_STREAM

On Wed, 2014-09-03 at 16:59 +0200, Jesper Dangaard Brouer wrote:
> Hi Eric,
> 
> When doing:
>  super_netperf 120 -H 192.168.8.2 -t UDP_STREAM -l 100 -- -m 256
> 
> I'm seeing function ip_idents_reserve() consuming most CPU.  Could you
> help, explain what is going on, and how I can avoid this?
> 
> Perf top:
>   11.67%  [kernel]   [k] ip_idents_reserve
>    8.37%  [kernel]   [k] fib_table_lookup
>    4.46%  [kernel]   [k] _raw_spin_lock
>    3.21%  [kernel]   [k] copy_user_enhanced_fast_string
>    2.92%  [kernel]   [k] sock_alloc_send_pskb
>    2.88%  [kernel]   [k] udp_sendmsg
> 

Because you use a single destination, all flows compete on a single
atomic to get their next IP identifier.

You can try to use netperf options  (-- -N -n) so that netperf uses
connected UDP sockets.

In this case, the IP identifier generator is held in each socket.




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