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RestoreLock is a method of the ReaderWriterLock class that accepts a reference to a LockCookie as its only parameter. RestoreLock blocks until it can acquire the required locks.
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RELEASELOCK AND RESTORELOCK
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The following instruction releases all locks that the current thread has on the RW instance of ReaderWriterLock:
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RW.ReleaseLock();
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Instead of using the ReleaseLock method, the following instructions perform roughly the same function:
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while(RW.IsReaderLockHeld) { RW.ReleaseReaderLock(); } while (RW.IsWriterLockHeld) { RW.ReleaseWriterLock(); }
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Since ReleaseLock returns a LockCookie structure we can save the current lock state for future use. During the period between ReleaseLock and RestoreLock, other threads have access to the values. This means that the values that are being protected by the ReaderWriterLock may have changed before RestoreLock is called. To handle this situation we can use the AnyWritersSince method we discussed in the previous section.
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RestoreLock is a method of the ReaderWriterLock class that accepts a reference to LockCookie as its only parameter. RestoreLock blocks until it can acquire the required locks.
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Listing 9.14 checks to see if some other thread has acquired a write lock since the ReleaseLock statement was executed.
Listing 9.14 The safe way to use ReleaseLock and RestoreLock (C#) Acquire a write lock Save the current WriterSeqNum
RW.AcquireWriterLock(Timeout.Infinite); int SeqNum = RW.WriterSeqNum; LockCookie Lock = RW.ReleaseLock(); . . .
RW.RestoreLock(ref Lock ); Restore the write lock if (RW.AnyWritersSince(SeqNum )) { Trace.WriteLine("A thread has written to the data"); } else { // Data has not changed since ReleaseLock } RW.ReleaseWriterLock();
Look for new writers
READER/WRITER LOCK
This is the safest way to use the release and restore lock methods. Failure to use the AnyWritersSince method may result in data values changing without the knowledge of the thread that uses RestoreLock. If the functionality of release and restore lock is required, use care to ensure that the ReaderWriterLock is not bypassed.
SUMMARY
We ve seen how a ReaderWriterLock can be used to allow multiple threads read access to a data element while preserving the integrity of the data. ReaderWriterLocks are a powerful construct that fit certain synchronization needs. When the situation is right, using a ReaderWriterLock can result in a marked performance increase. In the next chapter we examine the ThreadPool class. ThreadPools are collections of threads that are reused to perform some short-lived task. ThreadPools, like ReaderWriterLocks, can solve certain problems very well.
SUMMARY
The ThreadPool class
10.1 ThreadPool class and QueueUserWorkItem 182 10.2 The RegisterWaitForSingleObject method 184 10.3 Informational methods and properties 187 10.4 Two unsafe methods 190 10.5 The use of ThreadPools in .NET 192 10.6 Summary 193
A thread pool is a collection of threads tasked with units of work that exist for the life of the thread pool. It allows easy distribution of work among multiple threads and is an ideal construct for dealing with multiple short-lived tasks that can be performed in parallel. In this chapter we explore the powerful features of the .NET thread pool. For the examples in this chapter we revisit the matrix multiplication problem we discussed in chapter 8. The WorkManager class from chapter 8 was a simplified thread pool system. In the first section we will replace the thread control logic with a thread pool.
THREADPOOL CLASS AND QUEUEUSERWORKITEM
In chapter 8 we constructed a simplistic thread pool implementation; not surprisingly the result was somewhat complex. A thread pool allows for a simple means of performing multiple tasks on different threads. The thread pool manages its threads: It controls their number, lifetime, priority, and activities. The example in listing 10.1 demonstrates adding items for the ThreadPool to process using the QueueUserWorkItem method.
Listing 10.1
ThreadPool example (C#)
private void ThreadMethod(object O ) ThreadMethod { is invoked by Unit Work; the thread pool Work = (Unit)O; Work.DoWork(); ResultObjects.Add(Work); Interlocked.Decrement(ref ExpectedCompleteElements); if (ExpectedCompleteElements ==0) { FinishedWithWork.Set(); } } public void DoWork(Unit[] Work, WorkFinished Finished ) { ResultObjects.Clear(); Notify = Finished; ExpectedCompleteElements=Work.Length ; WaitCallback callback = new WaitCallback(ThreadMethod); for (long i = 0;i <Work.Length ;i++) { ThreadPool.QueueUserWorkItem(callback, Work[i]); QueueUserWorkItem } adds work elements } to the thread pool
There are several restrictions on thread pools. There can be only one thread pool per process. This means that if multiple tasks are being performed using thread pools in the same process, they will share the same thread pool. If one of the tasks takes a disproportionate share of the processing time, the other tasks will suffer. The reason that there is only one thread pool per process is that the method QueueUserWorkItem on the ThreadPool class is a static/shared method. The likely reason that the designers made this choice is to maximize performance.
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