Thanks for the reply.
I was not able to achieve the required task with the given pointers.
I ran the application with following command,
mpirun  -np 2 --mca btl_tcp_frag 9 --mca btl_tcp_max_send_size 8192 -host node-00,node-01 /home/atif/blah/aa_l

I still see the messages of size 65226 bytes. I am missing something? One side question, I cannot find documentation for the mca parameter btl_tcp_frag.

I am currently using openmpi version 1.2.2.

Best Regards,
Muhammad Atif

----- Original Message ----
From: George Bosilca <>
To: Open MPI Developers <>
Sent: Tuesday, April 1, 2008 1:31:46 AM
Subject: Re: [OMPI devel] limit tcp fragment size?

The btl_tcp_min_send_size is not exactly what you expect it to be. It 
drive only the send protocol (as implemented in Open MPI), and not the 
put protocol the TCP BTL is using.

You can achieve what you want with 2 parameters:
1. btl_tcp_frag set to 9. This will force the send protocol over TCP 
all the time
2. btl_tcp_max_send_size set to 16K which will define the size of a 
fragment in the pipelined send protocol.


On Mar 31, 2008, at 2:46 AM, Muhammad Atif wrote:
> G'day
> Just a quick basic question..... in case of tcp btl, how do I limit 
> the frag size?
> I do not want MPI to send a fragment of size greater than lets say 
> 16K in size.
> If I am not mistaken, should not the btl_tcp_min_send_size do the 
> trick?  If it is supposed to do it, why do i see packets of lenght 
> 64K ?
> Thanks in advance.
> Best Regards,
> Muhammad Atif
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