These are equivalent. #pragma omp parallel spawns a group of threads, while #pragma omp for divides loop iterations between the spawned threads. You can do both things at once with the fused #pragma omp parallel for directive.
Why Parallel.ForEach is not good for this task is explained in comments: it’s designed for CPU-bound (CPU-intensive) tasks. If you use it for IO-bound operations (like making web requests) – you will waste thread pool thread blocked while waiting for response, for nothing good. It’s possible to use it still, but it’s not best for … Read more
So this is a little more subtle, and requires some understanding of how the Gather collective places complex types. If you look at most examples of MPI_Gather, they’re of 1-d arrays, and it’s fairly easy to interpret what should happen; you’re getting (say) 10 ints from each process, and Gather is smart enough to put … Read more
A good way is to have the main array beeing alocated ouside the threads. Then you give to each thread the pointer to the part of the main array that should be computed by the thread. The following example is an implementation of a matrix multiplication (similar to dot for 2-D arrays) where: c = … Read more
(1) Nested parallelism in OpenMP: http://docs.oracle.com/cd/E19205-01/819-5270/aewbc/index.html You need to turn on nested parallelism by setting OMP_NESTED or omp_set_nested because many implementations turn off this feature by default, even some implementations didn’t support nested parallelism fully. If turned on, whenever you meet parallel for, OpenMP will create the number of threads as defined in OMP_NUM_THREADS. So, … Read more
The underlying database connections that the Entity Framework are using are not thread-safe. You will need to create a new context for each operation on another thread that you’re going to perform. Your concern about how to parallelize the operation is a valid one; that many contexts are going to be expensive to open and … Read more
To answer my own question, I found an example of a PausableThreadPoolExecutor in the javadocs of ThreadPoolExecutor itself. Here is my version using Guava’s Monitors: import com.google.common.util.concurrent.Monitor; import java.util.concurrent.ScheduledThreadPoolExecutor; import java.util.concurrent.ThreadFactory; public class PausableExecutor extends ScheduledThreadPoolExecutor { private boolean isPaused; private final Monitor monitor = new Monitor(); private final Monitor.Guard paused = new Monitor.Guard(monitor) { … Read more
Lazy<T> is a very good choice for Thread-Safe Initialization. I think it should be left to the consumer to provide that: var queue = new Lazy<ThreadSafeQueue<int>>(() => new ThreadSafeQueue<int>()); Parallel.For(0, 10000, i => { else if (i % 2 == 0) queue.Value.Enqueue(i); else { int item = -1; if (queue.Value.TryDequeue(out item) == true) Console.WriteLine(item); } … Read more
Does Java have support for multicore processors/parallel processing? Yes. It also has been a platform for other programming languages where the implementation added a “true multithreading” or “real threading” selling point. The G1 Garbage Collector introduced in newer releases also makes use of multi-core hardware. Java Concurrency in Practice Try to get a copy of … Read more