c# generating barcode Hybrid Thread Synchronization in VS .NET

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29 Hybrid Thread Synchronization
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I m not terribly fond of any of the thread synchronization constructs that use kernel-mode primitives because all of these primitives exist to block a thread from running, and threads are just too expensive to create and not have them run . Here is an example that hopefully clarifies the problem . Imagine a Web site into which clients make requests . When a client request arrives, a thread pool thread starts processing the client s request . Let s say that this client wants to modify some data in the server in a thread-safe way, so it acquires a reader-writer lock for writing . Let s pretend that this lock is held for a long time . As the lock is held, another client request comes in, so that thread pool creates a new thread for the client request and then the thread blocks trying to acquire the reader-writer lock for reading . In fact, as more and more client requests come in, the thread pool creates more and more threads and all these threads are just blocking themselves on the lock . The server is spending all its time creating threads so that they can stop running! This server does not scale well at all . Then, to make matters worse, when the writer thread releases the lock, all the reader threads unblock simultaneously and get to run, but now there may be lots of threads trying to run on relatively few CPUs, so Windows is context switching between the threads constantly . The result is that the workload is not being processed as quickly as it could because of all the overhead associated with the context switches . If you look over all the constructs shown in this chapter, many of the problems that these constructs are trying to solve can be much better accomplished using the Task class discussed in 26 . Take the Barrier class, for example: You could spawn several Task objects to work on a phase and then, when all these tasks complete, you could continue with one or more other Task objects . Compared to many of the constructs shown in this chapter, tasks have many advantages:
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Tasks use much less memory than threads and they take much less time to create and destroy . The thread pool automatically scales the tasks across available CPUs . As each task completes a phase, the thread running that task goes back to the thread pool where it can do other work if any is available for it . The thread pool has a process-global view of tasks and, as such, it can better schedule these tasks, reducing the number of threads in the process and also reducing context switching .
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Part V
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Reader-writer locks are very popular and useful .8 Trying to build one of these out of Task objects would be quite challenging . However, my Power Threading library includes a nonblocking read-writer class, which I call ReaderWriterGate . It looks like this (some methods are not shown):
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public sealed class ReaderWriterGate : IDisposable { public ReaderWriterGate(); public void Dispose(); public IAsyncResult BeginRead(ReaderWriterGateCallback callback, Object state, AsyncCallback asyncCallback, Object asyncState); public Object EndRead(IAsyncResult result); public IAsyncResult BeginWrite(ReaderWriterGateCallback callback, Object state, AsyncCallback asyncCallback, Object asyncState); public Object EndWrite(IAsyncResult result); }
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The ReaderWriterGateCallback is a delegate that looks like this:
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public delegate object ReaderWriterGateCallback(ReaderWriterGateReleaser releaser);
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The ReaderWriterGateReleaser class looks like this (some methods are not shown):
public sealed class ReaderWriterGateReleaser : IDisposable { public Object State { get; } // Returns the 'state' passed to BeginRead/BeginWrite public void Dispose(); }
The ReaderWriterGate makes a lock look just like an asynchronous I/O operation . In fact, my class even offers BeginRead and BeginWrite methods that take an AsyncCallback delegate and return an IAsyncResult, as well as EndRead and EndWrite methods that accept an IAsyncResult . I designed the class to work exactly like the APM that was discussed in 27 . Here s how it works . Put the code that requires read access to a resource in its own method and then call BeginRead, passing this method for the ReaderWriterGetCallback delegate parameter . When it is safe to read from the resource, the ReaderWriterGate object will have a thread pool thread invoke your method . Note that several thread pool threads could be executing methods that read from the resource concurrently . Put the code that requires write access to a resource in its own method and then call BeginWrite, passing this method for the ReaderWriterGetCallback delegate parameter . When it is safe to write to the resource, the ReaderWriterGate object will have a thread pool thread invoke your method . Note that the ReaderWriterGate object ensures that there will be only one thread pool thread executing a method that writes to the resource at a particular time .
Of course, if you only need mutual-exclusive access to a resource, you can always use a reader-writer lock and request only write access to the resource it protects .
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