Optimization of parallel programming

Question

I want to use MPI to make my program parallel, and I want to send something to other computers. I want to know which one is better: sending a huge buffer one time or sending two smaller messages 3 times atrent times during the execution instead of all at once?

Answer 1

I always try to send as much data as I can in a single MPI call. In my experience, sending many small bits of data greatly increases the overhead and network traffic, and I have even run into problems where I overwhelmed the computers' ability to keep up with the number of requests, because I was sending a large member of a complicated class, one integer at a time, to many workers. Therefore, when possible, send the entire data at once, unless you have some reason to believe it is too large.

Further, I strive to use 100% of all the CPU's my program claims. When you are working on shared resources, if you use a CPU, you need to actually use it. Otherwise, someone else who wants to use that core, or node, is blocked out while your program sits and does nothing. For example, on a Cray which I have used, even if you call for only two 'cores', the manager will reserve a full bank of 24 cores, essentially wasting 22. Or, perhaps one worker has nothing to do, while another chugs away -- again, wasting time. Hopefully, there is a way to balance the load, so to speak, to avoid unintentional waste of resources.

Back to the topic at hand. Demonstrate timing and efficiency of vector sending to yourself -- write a program which breaks up the vector into varying sizes of packets, and do the sends/receives. Test it with varying numbers of workers, and on several different configurations of computers, if you can. Before writing production code, do proof of concept, and what optimization you can. Test and time it!

Answer 2

It's almost always going to be faster to send the one big message than the smaller one. Each time you do a Send/Receive pair, the two processes have to go through the entire process of sending a message to each other, including at least 6 roundtrip messages. If you are just sending one larger message, there is a minimum of 2 roundtrip messages. Each of those messages can be very expensive (compared to doing things locally like packing all of your data into one buffer).

I'd encourage you to try it out both ways though to be sure that this applies to your application. It could be different if you're doing something unexpected.

Answer 3

Depending on your problem, sending all data may be more efficient, because the nodes have to be synced, every time. That may cause a delay.