Free-ebooks-library - Bahar Ali Khan

class Camera
{
public readonly int CameraID;
public Camera (int cameraID) { CameraID = cameraID; }
// Get image from camera: return a simple string rather than an image
public string GetNextFrame()
{
Thread.Sleep (123); // Simulate time taken to get snapshot
return "Frame from camera " + CameraID;
}
}
To obtain a composite image, we must call GetNextFrame on each of four camera
objects. Assuming the operation is I/O-bound, we can quadruple our frame rate with
parallelization—even on a single-core machine. PLINQ makes this possible with
minimal programming effort:
Camera[] cameras = Enumerable.Range (0, 4) // Create 4 camera objects.
.Select (i => new Camera (i))
.ToArray();
while (true)
{
string[] data = cameras
.AsParallel().AsOrdered().WithDegreeOfParallelism (4)
.Select (c => c.GetNextFrame()).ToArray();
Console.WriteLine (string.Join (", ", data)); // Display data...
}
GetNextFrame is a blocking method, so we used WithDegreeOfParallelism to get the
desired concurrency. In our example, the blocking happens when we call Sleep; in
real life it would block because fetching an image from a camera is I/O-intensive
rather than CPU-intensive.
Calling AsOrdered ensures the images are displayed in a consistent
order. Because there are only four elements in the sequence,
this would have a negligible effect on performance.
Changing the degree of parallelism
You can call WithDegreeOfParallelism only once within a PLINQ query. If you need
to call it again, you must force merging and repartitioning of the query by calling
IsParallel() again within the query:
"The Quick Brown Fox"
.AsParallel().WithDegreeOfParallelism (2)
.Where (c => !char.IsWhiteSpace (c))
.AsParallel().WithDegreeOfParallelism (3) // Forces Merge + Partition
.Select (c => char.ToUpper (c))
PLINQ | 885
Parallel
Programming