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-   -   Parallel Fluent: trouble going from 2 to 4 CPUs (http://www.cfd-online.com/Forums/fluent/42046-parallel-fluent-trouble-going-2-4-cpus.html)

Mario August 17, 2006 16:31

Parallel Fluent: trouble going from 2 to 4 CPUs
 
I started programming my CFD simulations in parallel to use with a Linux Cluster. I can run Serial simulations just fine, and I can run Parallel 2 CPU simulations fine as well. When I try to increase the number of CPUs higher than 2, the simulation halts when the iterations begin.

At first I thought it might be some of the parallel coding that I added, but I went back to the original serial version and I get the exact same result. It makes no difference if I spawn 4 nodes on 1 dual processor machine or spawn 1 node per CPU on 2 different dual processor machines (thought it might have been a communication issue).

Is there anything obvious that I should be looking for to fix in my UDF file? I am launching Fluent by:

fluent 2d -g -t4 -alnamd64

The -alnamd64 switch is needed for that particular cluster. I tried the switch -psmpi as well, but that didn't help.

Any suggestions?

cadaei August 17, 2006 16:52

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
try -pnmpi. you will need to use -cnf=hostname file path with the -pnmpi switch. hostname file is just text file with host names eg. node1 node2 etc

s.feven August 17, 2006 17:54

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
try partitioning the grid in serial before running in parallel

Mario August 18, 2006 16:56

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
Thanks for the responses. I tried the -pnmpi switch and I also tried partitioning the grid in serial before running in parallel, but neither seemed to help :(

Is there anything else (that I may have written in my UDF) obvious which would cause the solver to get stuck? I thought about File I/O, but I disabled any file read/write and that didn't help.

The UDF basically records velocities from particular cells (C_U,V,W), computes an output, and applies that value as a boundary input (F_PROFILE). I suppose I could try (as a test) getting rid of the UDF completely and see if Fluent still gets stuck....

Any more suggestions?

mAx August 21, 2006 05:21

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
hi try to spawn manually your nodes, and don't type -t4 option. It should be done before reading your case

Mario August 23, 2006 11:01

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
How do I manually spawn nodes from Fluent without the GUI? I've read Ch. 30 (Parallel Processing) of the Fluent Manual, but it only mentions spawning nodes through the GUI.

I created a test simulation which did not use any UDFs and I get the exact same results when going from 2 to 4 CPU's (simulation "locks up" upon beginning iterations - as if it is waiting for something....), so at least that narrows down the possibilities.

It is most likely a communication issue on the cluster itself, but I'm not sure how to get around it.....

mAx August 24, 2006 01:17

Re: Parallel Fluent: trouble going from 2 to 4 CPU
 
try with gui. you will know immediatly if your cluster is well set up. If it's ok, then write a journal while spawning your nodes. After that you will be able to spwan your nodes "manually" and automatically. It's like a -cnf options with hostfile


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