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t o ensure that, given the addition o f more and more processors, the throughput of the system improves commensurately (or at least doesn' t do the inverse of this) Because these are such large topics, we will tease them apart and review them in several subsequent chapters In this chapter, we stick to the general ideas, providing motivating examples as we go In 5, Windows Kernel Synchronization, we look at the foundational Windows kernel support used for synchronization, and then in 6, Data and Control Synchronization, we will explore higher level primitives available in Win32 and the NET Framework We won' t discuss per formance and scalability in great depth until 1 4, Performance and Scalability, although it's a recurring theme throughout the entire book
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Data Synchronization The solution to the general problem of data races is to serialize concurrent access to shared state Mutual exclusion is the most popular technique used to guarantee no two threads can be executing the sensitive region of instructions concurrently The sequence of operations that must be serial ized with respect to all other concurrent executions of that same sequence of operations is called a critical region Critical regions can be denoted using many mechanisms in today's sys tems, ranging from language keywords to API calls, and involving such ter minology as locks, mutexes, critical sections, monitors, binary semaphores, and, recently, transactions (see Further Reading, Shavit, Touitou) Each has its own subtle semantic differences The desired effect, however, is usually roughly the same So long as all threads use critical regions consistently to access certain data, they can be used to avoid data races Some regions support shared modes, for example reader/ writer locks, when it is safe for many threads to be reading shared data con currently We'll look at examples of this in 6, Data and Control Synchronization We will assume strict mutual exclusion for the discussion below What happens if multiple threads attempt to enter the same critical region at once If one thread wants to enter the critical region while another
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is already executing code inside, it must either wait until the thread leaves or it must occupy itself elsewhere in the meantime, perhaps checking back again sometime later to see if the critical region has become available The kind of waiting used differs from one implementation to the next, ranging from busy waiting to relying on Windows' support for waiting and signal ing We will return to this topic later Let's take a brief example Given some statement or compound state ment of code, S, that depends on shared state and may run concurrently on separate threads, we can make use of a critical region to eliminate the pos sibility of data races
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EnterCrit i c a l Region ( ) j Sj LeaveC rit i c a l Region ( ) j
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(Note that these APIs are completely fake and simply used for illustration) The semantics of the faux E nt e rC r it i c a l Region API are rather simple: only one thread may enter the region at a time and must otherwise wait for the thread currently inside the region to issue a call to L e a v e C r it i c a l
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Region This ensures that only one thread may be executing the statement
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at once in the entire process and, hence, serializes all executions It appears as if all executions of S happen atomically-provided there is no possibility of concurrent access to the state accessed in 5 outside of critical regions, and that 5 may not fail part-way through-although clearly 5 is not really atomic in the most literal sense of the word Using critical regions can solve both data invariant violations illustrated earlier, that is when 5 is ( * a ) ++, as shown earlier Here is the first problem
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