Operating System Fundamentals
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best answers from c, Lesson 2 C# Windows Forms
| Shared Memory Our first technique that we will try to introduce is the concept of shared memory. This is a very simple to understand concept but, unfortunately, not a very good method inmost cases. The term shared memory means that both threads have access to apiece of memory that they can both read and write to at the same time. If one thread puts a value into the memory location, then when the second thread looks at the memory it will see exactly the same thing. Of course, unless the two threads are executing on a multi-core processor, there will only be a single thread executing at a given instant. Configuring a variable to be shared between two processes on an operating system is usually possible. However, it is not particularly easy due to the security. The creation of shared variables between two threads of the same process is generally easy (in fact, too easy because a lot of mistakes are generally made by assuming. For the purpose of discussion we will assume that there is a variable called turn which is a simple integer that is available to both threadA and threadB. We will assume that the sharing is already setup. Now consider the following pseudocode: Configure shared variable turn Turn = 1 Start threadA Start threadB You might jump to the conclusion that threadA will be ahead of threadB because we asked it to start first. Keep in mind that the scheduler is responsible for picking the order. Just because you asked to start A first does not mean it actually had a chance to run Now lets try to modify the two threads to make use of the turn variable to make sure that threadA runs first then threadB. Operating System Fundamentals 39 threadA() { System.out.print(“Thread A System.out.println(); Turn = 2 } threadB() { While (turn == 1) ; System.out.print(“Thread B System.out.println(); } Let us take a quick look to see if this actually does what we want. Notice that we started by setting the shared variable called turn (both A and B can see this) to the value 1. When threadA is finished it sets the value to 2. If we look at threadB() we see that it looks at the value of turn and if it is equal to 1 it does nothing, but we expect that eventually threadA will finish and the value will beset to 2, allowing threadB to execute. This is good this provides us away that allows the two threads to synchronize. It does not matter if threadB gets to run for ten minutes before threadA even starts, because it will do nothing until threadA is finished. The variable called turn is an example of a synchronization variable. Although this solution works, it does pose a bit of a problem. Look again at threadB, we see that it starts with a loop that continuously checks the value called turn. Now if threadB were to run for five seconds, would the value of turn ever change Of course not, it is threadA that changes it. This type of checking is called polling and burns a lot of CPU time, which may consume more power from our battery (or may just cause the processor to heat up for no reason. Download 2.34 Mb. Share with your friends:
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