[Beowulf] Performance tuning for Jumbo Frames

[richard.walsh at comcast.net]

>On Dec 12, 2009 Håkon Bugge wrote: 
> 
>On Dec 12, 2009, at 7:59 , Rahul Nabar wrote: 
> 
>> I have seen a considerable performance boost for my codes by using 
>> Jumbo Frames. But are there any systematic tools or strategies to 
>> select the optimum MTU size? I have it set as 9000. (Of course, all 
>> switiching hardware supports jumbo frames and no talking to the 
>> external world required of the interfaces) Have you guys found 
>> performance to be MTU sensitive? 
> 
>Once (i.e. several years ago) I tested an application on Gbe and found 
>that around 1/3rd of the MTU was the optimum. But I guess YMMV. 


I would seem that a larger MTU would help in at least two situations, 
clearly applications with very large messages, but also those that 
have transmission bursts of messages below the MTU that could 
take advantage of hardware coalescing. The common MTU of 1500 was 
not chosen arbitrarily, but was probably not tuned to serve the "average" 
HPC application. If one is going to run one application predominately, 
then that application might prefer the default or not. If one is running 
a suite of codes, the it will probably help some and hurt others and 
should be "weighted average" tuned. 


I think the first-order focal point for performance should be on writing 
good code while choosing the "right" MTU size is a third order concern. 
Here is a stupid question ... it is the >>Maximum<< Transmission Unit ... 
Right? You don't get it all the time ... how is the run-time value set 
and adjusted based on the quality of message traffic? Experts ... ?? 


rbw 

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