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I have two Linux machines, each equipped with a Solarflare SFN5122F 10GbE NIC. The two NICs are connected together with an SFP+ Direct Attach cable.

I am using netperf to measure TCP throughput between the two machines. On one box, I run:


and on the other:

netperf -t TCP_STREAM -H 192.168.x.x -- -m 32768

I get:

MIGRATED TCP STREAM TEST from ( port 0 AF_INET to 192.168.x.x (192.168.x.x) port 0 AF_INET
Recv   Send    Send                          
Socket Socket  Message  Elapsed              
Size   Size    Size     Time     Throughput  
bytes  bytes   bytes    secs.    10^6bits/sec  

 87380  16384  32768    10.02    1321.34   

The measured throughput is 1.3Gb/s. This is 7.5x below the theoretical maximum, and only 30% faster than 1GbE.

What steps can I take to troubleshoot this?

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Did you enable jumbo frames? – gekkz Apr 10 '12 at 20:08
Do you get low throughput on UDP tests as well? – Shane Madden Apr 10 '12 at 20:10
@gekkz: Changing MTU to 9000 on both ends made no perceptible difference. – NPE Apr 10 '12 at 20:12
@ShaneMadden: UDP throughput is about the same. – NPE Apr 10 '12 at 20:12
What is the hardware of the boxes involved? You cannot achieve 10GbE line rate with anything less than modern Westmere/Nehalem/Sandy Bridge boxes. – pfo Apr 10 '12 at 20:55

few things:

  • did you try adjusting mtu to make use of jumbo-frames?
  • are you absolutely sure that the link between the two servers does not have any packet losses?
  • does ethtool show you any errors on the interfaces on both ends?
  • what does top/atop say during the prolonged test - do you see any of the cores fully occupied by iowait?

you'll [most probably] not achieve full 10gbit on a single tcp session but you can do some additional tuning to get closer to it by tweaking tcp handling and communication with the network card itself - take a look here or here .

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