netdev - RE: Linux stack performance drop (TCP and UDP) in 3.10 kernel in routed scenario

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Date:	Wed, 4 Jun 2014 14:12:41 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Suprasad Mutalik Desai' <suprasad.desai@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: Linux stack performance drop (TCP and UDP) in 3.10 kernel in
 routed scenario

From: Suprasad Mutalik Desai
>     Currently i am working on 3.10.12 kernel and it seems the Linux
> stack performance (TCP and UDP) has degraded drastically as compared
> to 2.6 kernel.
> 
> Results :
> 
> Linux 2.6.32
> ---------------------
> TCP traffic using iperf
>     - Upstream : 140 Mbps
>     - Downstream : 148 Mbps
> 
> UDP traffic using iperf
>     - Upstream : 200 Mbps
>     - Downstream : 245 Mbps
> 
> Linux 3.10.12
> --------------------
> TCP traffic using iperf
>     - Upstream : 101 Mbps
>     - Downstream : 106 Mbps
> 
> UDP traffic using iperf
>     - Upstream : 140 Mbps
>     - Downstream : 170 Mbps
> 
> Analysis:
> ---------------
> 1.   As per profiling data on Linux-3.10.12 it seems,
>              -   fib_table_lookup and ip_route_input_noref is being
> called most of the times and thus causing the degradation in
> performance.
> 
>     8.77    csum_partial 0x80009A20 1404
>     4.53    ipt_do_table 0x80365C34 1352
>     3.45    eth_xmit 0x870D0C88 5460
>     3.41    fib_table_lookup 0x8035240C 856    <----------
>     3.38    __netif_receive_skb_core 0x802B5C00 2276
>     3.07    dma_device_write 0x80013BD4 752
>     2.94    nf_iterate 0x802EA380 256
>     2.69    ip_route_input_noref 0x8030CE14 2520    <--------------
>     2.24    ip_forward 0x8031108C 1040
>     2.04    tcp_packet 0x802F45BC 3956
>     1.93    nf_conntrack_in 0x802EEAF4 2284
> 
> 2.    Based on the above observation, when searched,  it seems Routing
> cache code has been removed from Linux-3.6 kernel and thus every
> packet has to go       through ip_route_input_noref to find the
> destination.

That doesn't look like enough cpu time to give the observed reduction
in throughput (assuming those numbers are in %).

Are you sure the system is actually running at 100% cpu?
(Although I'm not sure what to trust for cpu usage with these sorts of tests.)

	David