netdev - Re: [net-next PATCH 1/1 V4] qdisc: bulk dequeue support for qdiscs with TCQ_F

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Date:	Thu, 25 Sep 2014 08:12:15 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Florian Westphal <fw@...len.de>,
	Dave Taht <dave.taht@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [net-next PATCH 1/1 V4] qdisc: bulk dequeue support for qdiscs
 with TCQ_F_ONETXQUEUE

On Thu, 2014-09-25 at 16:57 +0200, Jesper Dangaard Brouer wrote:

> We are saving 3% CPU, as I described in my post with subject:
> "qdisc/UDP_STREAM: measuring effect of qdisc bulk dequeue":
>  http://thread.gmane.org/gmane.linux.network/331152/focus=331154
> 
> Using UDP_STREAM on 1Gbit/s driver igb, I can show that the
> _raw_spin_lock calls are reduced with approx 3%, when enabling
> bulking of just 2 packets.
> 
> This test can only demonstrates a CPU usage reduction, as the
> throughput is already at maximum link (bandwidth) capacity.
> 
> Notice netperf option "-m 1472" which makes sure we are not sending
> UDP IP-fragments::
> 
>  netperf -H 192.168.111.2 -t UDP_STREAM -l 120 -- -m 1472
> 
> Results from perf diff::
> 
>  # Command: perf diff
>  # Event 'cycles'
>  # Baseline  Delta    Symbol
>  # no-bulk   bulk(1)
>  # ........  .......  .........................................
>  #
>      7.05%   -3.03%  [k] _raw_spin_lock
>      6.34%   +0.23%  [k] copy_user_enhanced_fast_string
>      6.30%   +0.26%  [k] fib_table_lookup
>      3.03%   +0.01%  [k] __slab_free
>      3.00%   +0.08%  [k] intel_idle
>      2.49%   +0.05%  [k] sock_alloc_send_pskb
>      2.31%   +0.30%  netperf  [.] send_omni_inner
>      2.12%   +0.12%  netperf  [.] send_data
>      2.11%   +0.10%  [k] udp_sendmsg
>      1.96%   +0.02%  [k] __ip_append_data
>      1.48%   -0.01%  [k] __alloc_skb
>      1.46%   +0.07%  [k] __mkroute_output
>      1.34%   +0.05%  [k] __ip_select_ident
>      1.29%   +0.03%  [k] check_leaf
>      1.27%   +0.09%  [k] __skb_get_hash
> 
> A nitpick is that, this testing were done on V2 of the patchset.
> 

You could avoid the fib_table_lookup() cost by using netperf -- -N -n

(connected UDP sockets)

And of course reduce message sizes to increase pps



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