Can I safely use OpenMP with C++11?

Question

The OpenMP standard only considers C++ 98 (ISO/IEC 14882:1998). This means that there is no standard supporting usage of OpenMP under C++03 or even C++11. Thus, any program that uses C++ >98 and OpenMP operates outside of standards, implying that even if it works under certain conditions, it's unlikely to be portable but definitely never guaranteed.

The situation is even worse with C++11 with its own multi-threading support, which very likely will clash with OpenMP for certain implementations.

So, how safe is it to use OpenMP with C++03 and C++11?

Can one safely use C++11 multi-threading as well as OpenMP in one and the same program but without interleaving them (i.e. no OpenMP statement in any code passed to C++11 concurrent features and no C++11 concurrency in threads spawned by OpenMP)?

I'm particularly interested in the situation where I first call some code using OpenMP and then some other code using C++11 concurrency on the same data structures.

Answer 1

Walter, I believe I not only told you the current state of things in that other discussion, but also provided you with information directly from the source (i.e. from my colleague who is part of the OpenMP language committee).

OpenMP was designed as a lightweight data-parallel addition to FORTRAN and C, later extended to C++ idioms (e.g. parallel loops over random-access iterators) and to task parallelism with the introduction of explicit tasks. It is meant to be as portable across as many platforms as possible and to provide essentially the same functionality in all three languages. Its execution model is quite simple - a single-threaded application forks teams of threads in parallel regions, runs some computational tasks inside and then joins the teams back into serial execution. Each thread from a parallel team can later fork its own team if nested parallelism is enabled.

Since the main usage of OpenMP is in High Performance Computing (after all, its directive and execution model was borrowed from High Performance Fortran), the main goal of any OpenMP implementation is efficiency and not interoperability with other threading paradigms. On some platforms efficient implementation could only be achieved if the OpenMP run-time is the only one in control of the process threads. Also there are certain aspects of OpenMP that might not play well with other threading constructs, for example the limit on the number of threads set by OMP_THREAD_LIMIT when forking two or more concurrent parallel regions.

Since the OpenMP standard itself does not strictly forbid using other threading paradigms, but neither standardises the interoperability with such, supporting such functionality is up to the implementers. This means that some implementations might provide safe concurrent execution of top-level OpenMP regions, some might not. The x86 implementers pledge to supporting it, may be because most of them are also proponents of other execution models (e.g. Intel with Cilk and TBB, GCC with C++11, etc.) and x86 is usually considered an "experimental" platform (other vendors are usually much more conservative).

OpenMP 4.0 is also not going further than ISO/IEC 14882:1998 for the C++ features it employs (the SC12 draft is here). The standard now includes things like portable thread affinity - this definitely does not play well with other threading paradigms, which might provide their own binding mechanisms that clash with those of OpenMP. Once again, the OpenMP language is targeted at HPC (data and task parallel scientific and engineering applications). The C++11 constructs are targeted at general purpose computing applications. If you want fancy C++11 concurrent stuff, then use C++11 only, or if you really need to mix it with OpenMP, then stick to the C++98 subset of language features if you want to stay portable.

I'm particularly interested in the situation where I first call some code using OpenMP and then some other code using C++11 concurrency on the same data structures.

There are no obvious reasons for what you want to not be possible, but it is up to your OpenMP compiler and run-time. There are free and commercial libraries that use OpenMP for parallel execution (for example MKL), but there are always warnings (although sometimes hidden deeply in their user manuals) of possible incompatibility with multithreaded code that give information on what and when is possible. As always, this is outside of the scope of the OpenMP standard and hence YMMV.