[Beowulf] very low performance for very small packets under MPICH (TCP_NODELAY?)

Mark Hahn hahn at physics.mcmaster.ca
Fri Dec 30 14:11:35 EST 2005


> 2.6.15) with MPI 1.2.7 and Gigabit ethernet with a 3COM Switch and
> 3C2000-T NIC cards. We detected a very low efficiency in communication

which appears to use the sk98lin driver.

> packets or so times are in the order of 0.03~secs. This degrades the
> average performance for very small packets by a factor of 100.

is this measurable on real code, or only benchmarks?  the reason I ask
is that real code probably doesn't generate a flood of tinygrams,
but the benchmark might, and if so would trigger interrupt mitigation. 

> Any pointers for understanding why the Nagle algorithm is still active

why have you jumped to the conclusion that Nagle is at fault here?

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