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This chapter is from the book

This chapter is from the book

Concurrency Made Simple

The purpose of the discussion, up to this point, has been to reframe the idea of concurrency. Concurrency is not a way to make a program run faster. It is not a complex juggling trick that ninja coders use to keep multiple balls in the air at one time. On the contrary, it is the apparent sequential execution of a program that is the complex trick. Sequential execution is an illusion maintained by a cabal of compiler writers and hardware architects. Concurrency is simply the relaxation of a fabricated constraint.

Threads

In the developer’s environment, where time and order are rigid and implicit constraints, “concurrency” is just another word for “order unspecified”—the way things are everywhere else. A concurrent program is just one that announces to an already-concurrent world that its correctness does not depend on the ordering of events that occur in separate components. In a concurrent program, those separate, partially ordered components are called threads of execution, or just threads. Within a thread, instructions are still executed in rigidly sequential order. The order of execution of instructions in two separate threads, however, is completely unspecified.

In the early days of computing, the choice of threads as a model for concurrency was not obvious. Developers that needed out-of-order processing were forced to brew their own concurrency constructs. Both literature and code from the 1960s contain a wide variety of models for asynchronous execution.

Threads probably originated in the late 1960s, with IBM’s OS/360. They were called “tasks,” and were an OS-level service that saved developers the trouble of building their own concurrency abstraction. In 1991 Java, called Oak at the time, adopted the thread model and supported it in the language, even on operating systems that did not.

Even today, threads are not the only model for concurrency. Languages such as Erlang, Go, and Clojure, for instance, each use an entirely different model.

Introducing threads into the programming model does not present an intrinsic problem. Operating two cars in parallel causes no problems unless they both try to occupy the same space at the same time. Similarly, operating two threads that are completely independent is also perfectly safe. There are millions of programs, each concurrently running in its own thread of execution on millions of separate computers, at this very moment. Most of these programs don’t interact with each other in any way and their behavior is perfectly well defined. The problems only arise when threads need to share state and resources.

Atomic Execution

When multiple threads change state that is accessible to both, the results can easily be nondeterministic. Because there is no relationship between the order in which statements are executed, subtle changes in timing can change the result of running the program.

Consider the following code:

executionCount++;
someTask();

Just by inspection, it seems likely that the variable executionCount is meant to count the number of times that the function someTask is called. In a concurrent environment, however, this code, as it stands, does not have deterministic behavior because the ++ operation is not atomic—it is not a single, indivisible action. Table 1.1 demonstrates an execution sequence that fails to record one execution.

Table 1.1 Non-Atomic Execution

executionCount 1/2 4

Thread 1

Thread 2

read execution count (4)

read execution count (4)

increment (5)

increment (5)

store execution count (5)

store execution count (5)

call someTask

call someTask

Synchronization is the basic mechanism through which multiple Java threads share state in such a way that the result of the interaction is deterministic. In itself, synchronization is a simple idea: a critical section of code is protected by a mutual-exclusion lock or mutex. When a thread enters the critical section—that is, it begins executing instructions from it—it is said to seize the mutex. No other thread can enter the section until the first thread leaves.

The previous code becomes deterministic if only a single thread is allowed to execute the critical section at any given time:

synchronized(this) {
    executionCount++;
    someTask();
}

Synchronization is the crux of creating correct concurrent Java programs, and is the basis for a lot of things that are definitely not simple. Those things make up the content of the rest of this book.

Visibility

There is one more thing, however, that is simple. Remember that the previous example is an abstraction! It is written in a computer language—Java, in this case—and is, therefore, related to the actual behavior of hardware only by the grace of compiler writers, JVM developers, and hardware architects. Those two Java statements translate into hundreds of microinstructions, many of them executed in parallel, over tens of hardware clock cycles. The illusion that there are two statements, happening in order, is no more than an illusion.

Maintaining the illusion is not something that near-the-metal developers do naturally. On the contrary, they find sequential programs naive, clumsy, and wasteful. They are only too happy to fix them by re-ordering instructions, executing multiple instructions in parallel, representing a single piece of program state as multiple copies, and so on. By doing so, they do their very best to make optimal use of the immense power of the multiple processors that comprise even the tiny devices we carry in our pockets.

In general, we’re glad to have them perform these optimizations. They make our programs run faster, on multiple hardware platforms, using tricks in which application developers are just not that interested. There is one important condition on this optimization, however: They must not break the illusion of sequentiality! In other words, compilers and hardware pipelines can reorder and parallelize all they want to optimize the code, as long as developers can’t tell that they did it.

In making a program concurrent, a developer clearly states that there is no sequential dependency between the states controlled by different threads. If there is no sequential dependency, a compiler should be free to perform all sorts of optimizations that would otherwise have been unsafe. Without an explicit ordering between events in different threads, the compiler is free to make changes in the execution sequence of one thread without any consideration of the statements in any other.

A correct concurrent program is one that abides by the contract for maintaining an illusion. Negotiation between application programmers and hardware developers produce a language, and that language is a contract. The application developers get their illusion of sequential execution, something about which they can reason. The hardware developers get a toolbox of clever tricks they can use to make programs run fast. In the middle is the contract.

In Java, that contract is called the memory model. On one side of the contract is the application programmer, reasoning about her program in a high-level language. On the other side of the contract are the compiler writers, virtual machine developers, and hardware architects, moving everything that isn’t explicitly forbidden. Developers who talk about hardware when discussing concurrency are missing the point. A correct concurrent program is not about hardware; it is about developers keeping their end of the contract.

Fortunately, in Java, the contract is easily stated. The following single sentence states it almost completely:

Whenever more than one thread accesses a given state variable, and one of them might write to it, they all must coordinate their access to it using synchronization.

  • (Göetz, et al. 2006)

A correct concurrent Java program is one that abides by this contract—no more, no less. Note in particular that whether a thread reads or writes mutable state does not affect its need for synchronization in any way.

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