C++/g++: Concurrent programm

Question

Hi,

I got a C++ program (source) that is said to work in parallel. However, if I compile it (I am using Ubuntu 10.04 and g++ 4.4.3) with g++ and run it, one of my two CPU cores gets full load while the other is doing "nothing".

So I spoke to the one who gave me the program. I was told that I had to set specific flags for g++ in order to get the program compiled for 2 CPU cores. However, if I look at the code I'm not able to find any lines that point to parallelism.

So I have two questions:

• Are there any C++-intrinsics for multithreaded applications, i.e. is it possible to write parallel code without any extra libraries (because I did not find any non-standard libraries included)?
• Is it true that there are indeed flags for g++ that tell the compiler to compile the program for 2 CPU cores and to compile it so it runs in parallel (and if: what are they)?

Answer 1

AFAIK there are no compiler flags designed to make a single-threaded application exploit parallelism (it's definitely a nontrivial operation), with the exception of parallelization of loops iterations (-ftree-parallelize-loops), that, still, must be activated carefully; still, even if there's no explicit threads creation, there may be some OpenMP directives to parallelize several instruction sequences.

Answer 2

The current C++ language standard has no support for multi-processing in the language or the standard library. The proposed C++0x standard does have some support for threads, locks etc. I am not aware of any flags for g++ that would magically make your program do multi-processing, and it's hard to see what such flags could do.

Answer 3

Look for the occurrence of "thread" and/or "std::thread" in the source code.

Answer 4

The only thing I can think of is openMosix or LinuxPMI (the successor of openMosix). If the code uses processes then process "migration" technique makes is possible to put processes at work on different machines (which have the specified linux distribution installed).

Check for threads (grep -i thread), processes (grep fork) in your code. If none of this exists, then check for MPI. MPI requires some extra configuration since I recall (only used it for some homeworks in faculty).

As mentioned gcc (and others) implements some ways of parallelism with OpenMP with some pragmas.