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Old 07-02-2016, 03:45 PM   #1
James Hopper
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Linux box Throughput Degradation


Hey Everyone,
I have connected two Ubuntu boxes via OVS running on CentOS 6.7. I am facing unpredictable data transfer rates. Both the end hosts are connected with OVS via 1 Gbps links. I am using Iperf for data transmission between these boxes. Sometimes I achieve ~1 Gbps and sometimes the throughput drops to ~430 Mbps.
Any insight as to what might be the potential issue?
 
Old 07-02-2016, 04:10 PM   #2
biosboy4
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Linux box Throughput Degradation

when you ping one host from the other and vise versa, let it run for 30 min or so and see how much your latency fluctuates.

You can then temporarily swap out the hosts with 2 new, known to be fine boxes/vms/whatevertheserversare in the same vlan/ip and re-do tue ping test. if the tests are identical then it is likely a network problem.

Edit: that is IF the ping test shows an issue. If not, look elsewhere.

Last edited by biosboy4; 07-02-2016 at 04:12 PM.
 
Old 07-02-2016, 04:21 PM   #3
James Hopper
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Hey biosboy many thanks for your response.

Ping is producing almost constant RTT. The OVS box has multiple slots and I have tried different Src, Dst boxes but the issue persist. Any idea how this issue can be localized. Just to add, some of the interfaces are USB 3.0 to Gb ethernet while 1 is PCI Gigabit ethernet and another is PCIe Gigabit ethernet.

The key observation is that rate does not change during the life of the flow. Rather it persist to the initial rate which is either ~430 Mbps or 1Gbps..
 
Old 07-02-2016, 11:52 PM   #4
jnihil
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Try using UDP instead of TCP (default) for iperf.
I've always found it to be a better method as TCP will back-off on packet loss where UDP will not.
 
Old 07-03-2016, 12:25 AM   #5
James Hopper
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Should not TCP achieve maximum throughput when there is only 1 long running flow. It appears it is achieving but in certain cases. The issue does not seem to be TCP window related. Nevertheless, I tried UDP and got the similar behavior. Any idea if some auto setting of device or kernel playing a role here?
 
Old 07-03-2016, 04:27 AM   #6
jnihil
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Using UDP in iperf is a one way stream at full speed.
Performance of tcp is still limited due to the acknowledgement packets creating some latency.

Having said that, what you're seeing is strange. I'm suspecting OVS.
I haven't much experience with OVS, but have tried the same test using a simple Linux bridge instead?
 
Old 07-03-2016, 12:24 PM   #7
James Hopper
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Hey Jnihil
Many thanks. The issue is resolved. One of the interfaces of the switch was causing the problem.
 
Old 07-03-2016, 02:18 PM   #8
biosboy4
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Linux box Throughput Degradation

Makes sense, might eant to make sure your vlans and the rest of your network flow to figure out if it is a hardware or co figuration issue.
 
  


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