18: Multithreading in C#.NET

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18: Multithreading
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programming However, considerable complexity remains, not so much in writing a program that has multiple threads, but in doing so in a manner that maintains atomicity, avoids deadlocks, and does not introduce execution uncertainty such as race conditions
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Atomicity
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Consider code that transfers money from a bank account First, the code verifies whether there are sufficient funds; if there are, the transfer occurs If after checking the funds, execution switches to a thread that removes the funds, an invalid transfer may occur when execution returns to the initial thread Controlling account access so that only one thread can access the account at a time fixes the problem and makes the transfer atomic An atomic operation is one that either completes all of its steps fully, or restores the state of the system to its original state A bank transfer should be an atomic operation because it involves two steps In the process of performing those steps, it is possible to lose operation atomicity if another thread modifies the account before the transfer is complete Identifying and implementing atomicity is one of the primary complexities of multithreaded programming The complexity increases because the majority of C# statements are not necessarily atomic _Count++, for example, is a simple statement in C#, but it translates to multiple instructions for the processor 1 The processor reads the data in Count 2 The processor calculates the new value 3 Count is assigned a new value (even this may not be atomic) After the data is accessed, but before the new value is assigned, a different thread may modify the original value (perhaps also checking the value prior to modifying it), creating a race condition because the value in Count has, for at least one thread s perspective, changed unexpectedly
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To avoid such race conditions, languages support the ability to restrict blocks of code to a specified number of threads, generally one However, if the order of lock acquisition between threads varies, a deadlock could occur such that threads freeze, each waiting for the other to release its lock
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18: Multithreading
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Thread A Acquires a lock on a
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Thread B Acquires a lock on b Requests a lock on a Deadlocks, waiting for a
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Requests a lock on b Deadlocks, waiting for b
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At this point, each thread is waiting on the other thread before proceeding, so each thread is blocked, leading to an overall deadlock in the execution of that code
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The problem with code that is not atomic or causes deadlocks is that it depends on the order in which processor instructions across multiple threads occur This dependency introduces uncertainty concerning program execution The order in which one instruction will execute relative to an instruction in a different thread is unknown Many times, the code will appear to behave uniformly, but occasionally it will not, and this is the crux of multithreaded programming Because such race conditions are difficult to replicate in the laboratory, much of the quality assurance of multithreaded code depends on long-running stress tests and manual code analysis/reviews
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12 discussed delegates and events Programming multiple threads with C# depends heavily on the syntax of delegates In order to start a new thread, it is necessary to call a delegate that contains the code for the separate thread Listing 181 provides a simple example, and Output 181 shows the results
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Listing 181: Starting a Method in a Separate Thread
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using System; using SystemThreading; public class RunningASeparateThread {
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public const int Repetitions = 1000; public static void Main() { ThreadStart threadStart = new ThreadStart(DoWork); Thread thread = new Thread(threadStart); threadStart(); for (int count = 0; count < Repetitions; count++) { ConsoleWrite('-'); } threadJoin(); } public static void DoWork() { for (int count = 0; count < Repetitions; count++) { ConsoleWrite(''); } } }
OUTPUT 181:
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
18: Multithreading
The code that is to run in a new thread appears in the DoWork() method This method prints to the console repeatedly during each iteration within a loop Besides the fact that it contains code for starting another thread, the Main() method is virtually identical in structure to DoWork(), except that it displays - The resultant output from the program is a series of dashes until the thread context switches, at which time the program displays periods until the next thread switch, and so on (On Windows, it is possible to increase the chances of a thread context switch by using Start /low /b <programexe> to execute the program This will assign the entire process a lower priority, causing its threads to be interrupted more frequently, and thus causing more frequent thread switches)
Starting a Thread In order for DoWork() to run under the context of a different thread, you must first instantiate a SystemThreadingThreadStart delegate around the DoWork() method Next, you pass the ThreadStart delegate instance to the SystemThreadingThread constructor before commencing execution of the thread with a call to threadStart() In Listing 181, you instantiate the thread in two separate steps, explicitly instantiating a SystemThreadingThreadStart instance and assigning it to a variable before instantiating the SystemThreadingThread object As Listing 182 demonstrates, you could combine both statements, or you could use C# 20 s delegate inference to avoid any explicit syntax to instantiate ThreadStart and instead pass DoWork directly into SystemThreadingThread s constructor