qr code c# open source Exploring the C# Library in Visual C#

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Exploring the C# Library
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stated, in this case, the transformation is arbitrary (being used simply for demonstration) The For( ) automatically breaks up the calls to MyTransform( ) so they can be run on separate portions of data in parallel Therefore, if you run this program on a computer that has two or more available processors, the For( ) loop can run in parallel It is important to understand that not all loops will be more efficient when parallelized In general, small loops or loops that perform very simple operations are often faster as sequential rather than parallel loops This is why the for loop that initializes data is not a parallel For( ) The reason that small or very simple loops might not be efficient when parallelized is because the time needed to set up the parallel tasks and the time needed to context-switch exceeds the time saved by parallelization To prove this point, the following program creates both parallel and sequential versions of both for loops in the program and times each one for comparison purposes:
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// Show timing differences between sequential and parallel for loops using System; using SystemThreadingTasks; using SystemDiagnostics; class DemoParallelFor { static int[] data; // A method to be run as the body of a parallel loop // The statements in this loop are designed to simply // consume some CPU time for the purposes of demonstration static void MyTransform(int i) { data[i] = data[i] / 10; if(data[i] if(data[i] if(data[i] if(data[i] } static void Main() { ConsoleWriteLine("Main thread starting"); // Create a Stopwatch instance to time loops Stopwatch sw = new Stopwatch(); data = new int[100000000]; // Initialize data swStart(); // Parallel version of initialization loop ParallelFor(0, dataLength, (i) => data[i] = i ); swStop(); ConsoleWriteLine("Parallel initialization loop: swElapsedTotalSeconds); {0} secs", < > > > 1000) data[i] = 0; 1000 & data[i] < 2000) data[i] = 100; 2000 & data[i] < 3000) data[i] = 200; 3000) data[i] = 300;
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M u l t i t h r e a d i n g , P a r t Tw o : E x p l o r i n g t h e Ta s k P a r a l l e l L i b r a r y a n d P L I N Q
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swReset(); swStart(); // Sequential version of initialization loop for(int i=0; i < dataLength; i++) data[i] = i; swStop(); ConsoleWriteLine("Sequential initialization loop: {0} secs", swElapsedTotalSeconds); ConsoleWriteLine();
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PART II
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// Perform transforms swStart(); // Parallel version of transformation loop ParallelFor(0, dataLength, MyTransform); swStop(); ConsoleWriteLine("Parallel transform loop: swElapsedTotalSeconds); swReset(); swStart(); // Sequential version of transformation loop for(int i=0; i < dataLength; i++) MyTransform(i); swStop(); ConsoleWriteLine("Sequential transform loop: {0} secs", swElapsedTotalSeconds); ConsoleWriteLine("Main thread ending"); } } {0} secs",
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The following output was produced using a dual-core processor:
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Main thread starting Parallel initialization loop: 10537757 secs Sequential initialization loop: 03457628 secs Parallel transform loop: 42246675 secs Sequential transform loop: 53849959 secs Main thread ending
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First, notice that the parallel version of the initialization loop ran about three times slower than the sequential version This is because (in this case) assignment takes so little time that the overhead added by parallelism exceeds the gains Now, notice that the parallel
Part II:
Exploring the C# Library
transform loop ran faster than its sequential equivalent In this case, the gains of parallelization more than offset the overhead added by parallelization
NOTE In general, you should consult Microsoft s current guidelines in regards to what types of
loops make the best use of parallelization You will also want to confirm that you are actually getting performance gains before using a parallel loop in released application code There are a couple of other things to mention about the preceding program First, notice that the parallel initialization loop uses a lambda expression to initialize data It is shown here:
ParallelFor(0, dataLength, (i) => data[i] = i );
Here, the body of the loop is specified by a lambda expression (Again, recall that a lambda expression creates an anonymous method) Thus, there is no requirement that For( ) be used with a named method The second point of interest is the use of the Stopwatch class to handle the loop timing This class is in SystemDiagnostics To use Stopwatch, create an instance and then call Start( ) to begin timing and Stop( ) to end timing Use Reset( ) to reset the stopwatch There are various ways to obtain the duration The approach used by the program is the Elapsed property, which returns a TimeSpan object Using the TimeSpan object, the seconds (including fractional seconds) are displayed by use of the TotalSeconds property As this program shows, Stopwatch is very useful when developing parallel code As mentioned, the For( ) method returns an instance of ParallelLoopResult This is a structure that defines the following two properties: public bool IsCompleted { get; } public Nullable<long> LowestBreakIteration { get; } IsCompleted will be true if the loop completed all requested iterations In other words, it is true if the loop ran normally It will be false if the loop was terminated early LowestBreakIteration contains the lowest value of the loop control variable if the loop was terminated early via a call to ParallelLoopStateBreak( ) To have access to a ParallelLoopState object, you must use a form of For( ) whose delegate takes a second parameter that receives the current loop state Here is the simplest one: public static ParallelLoopResult For(int fromInclusive, int toExclusive, Action<int, ParallelLoopState> body) In this version, the Action delegate that describes the body of the loop is defined like this: public delegate void Action<in T1, in T2>(T arg1, T2 arg2) For use with the For( ), T1 must be int and T2 must be ParallelLoopState Each time this delegate is called, the current loop state is passed to arg2 To stop a loop early, call Break( ) on the ParallelLoopState instance inside body Break( ) is defined as shown here: public void Break( )
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M u l t i t h r e a d i n g , P a r t Tw o : E x p l o r i n g t h e Ta s k P a r a l l e l L i b r a r y a n d P L I N Q
A call to Break( ) requests that the parallel loop stop as soon as possible, which might be a few iterations beyond the one in which Break( ) is called However, all iterations prior to the one in which Break( ) is called will still execute Also, remember that portions of the loop might be running in parallel, so if 10 iterations have taken place, it does not necessarily mean that those 10 iterations represent the first 10 values of the loop control variable Breaking from a parallel For( ) loop is often useful when data is being searched If the desired value is found, there is no need to further execute the loop It might also be used if invalid data is encountered during an operation The following program demonstrates the use of Break( ) with a For( ) loop It reworks the previous example so that MyTransform( ) now has a ParallelLoopState parameter and the Break( ) method is called if a negative value is found in data Inside Main( ), a negative value is put into the data array (which will cause the loop to break) The completion status of the transform loop is checked Since the negative value in data will cause it to terminate early, the IsCompleted property will be false, and the iteration count at which the loop was terminated is displayed (The program removes the redundant loops used by the previous version, keeping only the most efficient of each, which is the sequential initialization loop and the parallel transform loop)
// Use ParallelLoopResult, ParallelLoopState, and Break() // with a parallel For loop using System; using SystemThreadingTasks; class DemoParallelForWithLoopResult { static int[] data; // A method to be run as the body of a parallel loop // The statements in this loop are designed to simply // consume some CPU time for the purposes of demonstration static void MyTransform(int i, ParallelLoopState pls) { // Break out of loop if a negative value is found if(data[i] < 0) plsBreak(); data[i] = data[i] / 10; if(data[i] if(data[i] if(data[i] if(data[i] } static void Main() { ConsoleWriteLine("Main thread starting"); data = new int[100000000]; // Initialize data < > > > 1000) data[i] = 0; 1000 & data[i] < 2000) data[i] = 100; 2000 & data[i] < 3000) data[i] = 200; 3000) data[i] = 300;
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