Even if your program benefits from using OpenMP, you won’t see it because you are measuring the wrong time. clock() returns the total CPU time spent in all threads. If you run with four threads and each runs for 1/4 of the time, clock() will still return the same value since 4*(1/4) = 1. You … Read more
There’s a few things going on here, that come down to: You have to work fairly hard to get every last bit of performance out of the memory subsystem; and Different benchmarks measure different things. The first helps explain why you need multiple threads to saturate the available memory bandwidth. There is a lot of … Read more
I’ll post here what I posted to Concurrent random number generation : I think you’re looking for rand_r(), which explicitly takes the current RNG state as a parameter. Then each thread should have its own copy of seed data (whether you want each thread to start off with the same seed or different ones depends … Read more
The gcc command in the latest Xcode suite is no longer the GCC frontend to LLVM (based on the very old GCC 4.2.1) but rather a symlink to clang. Clang does not (yet) support OpenMP. You have to install separately another version of GCC, e.g. by following this tutorial or by using any of the … Read more
CMake has a standard module for testing if the compiler supports OpenMP: find_package(OpenMP) if (OPENMP_FOUND) set (CMAKE_C_FLAGS “${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}”) set (CMAKE_CXX_FLAGS “${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}”) set (CMAKE_EXE_LINKER_FLAGS “${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}”) endif() Note: This answer is not recommended to be used anymore for including OpenMP in the project for current CMake versions. Refer to the other answers.
It seems to me as though the clock () function is calculating each thread’s individual time and adding up them up and displaying them. This is exactly what clock() does – it measures the CPU time used by the process, which at least on Linux and Mac OS X means the cumulative CPU time of … Read more
The difference between tasks and sections is in the time frame in which the code will execute. Sections are enclosed within the sections construct and (unless the nowait clause was specified) threads will not leave it until all sections have been executed: [ sections ] Thread 0: ——-< section 1 >—->*—— Thread 1: ——-< section … Read more
The effect on g_qCount is the same, but what’s done is different. An OpenMP critical section is completely general – it can surround any arbitrary block of code. You pay for that generality, however, by incurring significant overhead every time a thread enters and exits the critical section (on top of the inherent cost of … Read more
Update: OpenMP 5.0 defines the interactions to C++11 and further. Among others, it says that using the following features may result in unspecified behavior: Data-dependency ordering: atomics and memory model Additions to the standard library C++11 library So clearly, mixing C++11 atomics and OpenMP 5.0 will result in unspecified behavior. At least the standard itself … Read more
Others have since answered most of the question but I would like to point to some specific cases where a particular scheduling type is more suited than the others. Schedule controls how loop iterations are divided among threads. Choosing the right schedule can have great impact on the speed of the application. static schedule means … Read more