[Beowulf] Performance tuning for Jumbo Frames

Patrick Geoffray patrick at myri.com
Sat Dec 12 13:45:35 EST 2009

Hi Richard,

richard.walsh at comcast.net wrote:
> 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.

Such coalescing is typically not done in hardware. TCP with Nagle will 
coalesce small messages going to the same destination. However, Nagle 
trades latency for bandwidth, so latency junkies such as HPC apps often 
disable Nagle when using TCP. Some MPI implementations do coalesce small 
messages, mainly to look good on packet rate benchmarks.

> The common MTU of 1500 was not chosen arbitrarily,

Never underestimate what a standard committee can do.

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