Sorry I previously missed this mail.
I just recently added an MCA parameter to OMPI that allows enabling
or disabling Nagle's algorithm on the TCP BTL (OMPI's lowest layer
for MPI point-to-point communications for TCP). However, I'll tell
you that some of the OMPI developers thought that this was a useless
MCA parameter and that all users should leave Nagle's algorithm
disabled because MPI performance is usually concerned about the
latency -- and when it's not about latency, the extra bandwidth you
get from Nagle is so small that it won't matter.
This new MCA parameter is btl_tcp_use_nagle, and it accepts values of
0 or 1. It will show up in Open MPI v1.3 (or you can get a nightly
trunk tarball and try it out there).
We do not have a fine-grained mechanism to select to use Nagle for
some sockets and not for other sockets, sorry...
On Jul 5, 2007, at 8:26 AM, Biagio Cosenza wrote:
Hi,
I'm using Open MPI in a real time rendering system and I need an
accurate latency control.
consider the 'Nagle' optimization implemented in the TCP/IP
protocol, which delays small packets for a short time period to
possibly combine them with successive packets generating network
friendly packet sizes.
This optimization can result in a better throughput when lots of
small packets are sent, but can also lead to considerable
latencies, if packets get delayed several times.
For example, I want to turn the Nagle optimization on for sockets
in which updated scene data is streamed to the clients, as
throughput is the main issue here.
On the other hand, I want turn it off for e.g. sockets used to send
tiles to the clients, as this has to be done with an absolute
minimum of latency.
Can I do it with OpenMPI?
Am I using the the wrong tool?
Thanks in advance
Biagio Cosenza
an italian MSc student
Università di Salerno
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Jeff Squyres
Cisco Systems
