Re: [Discussion] "Slowest link" reported at 10G when it is 1G

[Richard Carlson <rcarlson@xxxxxxxxxxxxx>]

Hi Christine;

This is a byproduct of a Gigabit Ethernet adapter doing interrupt 
coalescing.  The link detection system uses packet pair timings to 
determine the bottleneck link speed.  Each packet is timestamped by the 
appropriate Libpcap function when the packet is processed by the 
kernel.  With interrupt coalescing, the NIC holds onto the packets until 
several have arrived and then it generates a single interrupt to the 
kernel.  This reduces the load on the CPU, but it means that the packets 
timestamps are measuring how fast the packets can be transferred across the 
systems' memory bus.  Not the how fast they arrive at the interface.

The simple workaround is to look at the documentation for your NIC in the 
kernel's Documentation/networking directory.  Turning off the interrupt 
coalescing option will increase the CPU load, but the NDT server will 
correctly report the bottleneck link capacity.  Hopefully I can develop a 
better solution in the near future.

Regards;
Rich

At 04:42 PM 12/8/2006, Christine Siegel wrote:
> Now that we have our NDT server up and running, testing using a 
> workstation with a 1G connection to the network is reported by NDT to have 
> a "slowest link" of 10G (OC-192). Can you explain?
>
> Thanks,
> Chris Siegel
>
> --
> Christine Siegel
> Systems Administrator
> University Systems & Security
> Fairleigh Dickinson University
> Email: siegel@xxxxxxx
> Voice: (201) 692-7131
>
>
> _______________________________________________
> Discussion mailing list
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> http://internal.web100.org/mailman/listinfo/discussion

------------------------------------



Richard A. Carlson                              e-mail: RCarlson@xxxxxxxxxxxxx
Network Engineer                                phone:  (734) 352-7043
Internet2                                       fax:    (734) 913-4255
1000 Oakbrook Dr; Suite 300
Ann Arbor,  MI  48104 


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