On May 19, 2016, at 12:33 PM, Guo, Yanfei <yguo@anl.gov> wrote:

Hi Alex,


On 5/18/16, 6:26 PM, "Alexandre Vieira" <nullpt@gmail.com> wrote:

Hello list,


I'm currently writing a paper on distributed computing and also elaborate a simple use case to include as a proof of concept.


My setup:


- 3 VMs running on different machines (same HW) with 1 CPU and 2GB RAM running FreeBSD 10.3
- Connected through a 1Gbps LAN and communicate passwordless through SSH.

- MPICH 3.2_1 installed and working on the three nodes


I've tried to compile and run some code samples that are available for mpich (i.e:
http://people.sc.fsu.edu/~jburkardt/c_src/mpi/search_mpi.c)  but I always end up getting worst times running on the 3 nodes that I get running on 1 node alone, even when I can get all
3 nodes to get up to a 100% CPU usage for a couple of minutes.

How much slower? When you say running on 1 node, do you mean all three VMs are on the same physical machine, or just running a serial implementation of the program? How many CPU cores does the physical machine have?

I know there is network overhead and SSH isn't helping either, so without investing too much time I unfortunately won't have, I'd like to ask you what kind of simple program could I write just for the sake of a proof of concept
that running a program with 3 nodes would be faster than running on one.

Network latency could be a problem because the communication goes through memory when all processes/VMs are on the same physical machine, which is way faster then Ethernet.

Maybe dealing with slower operations like disk I/O do counter the inter-node communication?

It is unlikely that disk I/O has something to do with this.

Many thanks for your kind help


BR
Alex

Yanfei Guo
Postdoctoral Researcher
MCS Division, ANL


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