[Beowulf] Performance tuning for Jumbo Frames
patrick at myri.com
Sat Dec 12 13:45:35 EST 2009
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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