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From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-07-25 08:17:21

On Jul 23, 2007, at 8:53 PM, Jeff Squyres wrote:

> It looks like we enable Nagle right when TCP BTL connections are
> made. Surprisingly, it looks like we don't have a run-time option to
> turn it off for power-users like you who want to really tweak around.

I should note that I got the logic backwards here (@#@#$@#$!!! I'm
going to claim it was too early in the morning when I initially
replied...). Open MPI *disables* Nagle's algorithm by default. In
hindsight, this is exactly what you would expect since OMPI is trying
to reduce latency.

Setting optval to 1 enables NODELAY, which *disables* Nagle's algorithm.
Setting optval to 0 disables NODELAY, which *enables* Nagle's algorithm.

I'm adding a new MCA parameter named "btl_tcp_use_nagle" which will
make this a bit easier to set (no negative logic for the user to
figure out) in a future release -- probably the 1.3 series.

Jeff Squyres
Cisco Systems