算法导论的 并行算法

Thomas H. Cormen Charles E. Leiserson Ronald L. Rivest Clifford Stein Introduction to Algorithms Third Edition The MIT Press Cambridge, Massachusetts London, England 27 Multithreaded Algorithms The vast majority of algorithms in this book are serial algorithms suitable for running on a uniprocessor computer in which only one instruction executes at a time. In this chapter, we shall extend our algorithmic model to encompass parallel algorithms, which can run on a multiprocessor computer that permits multiple instructions to execute concurrently. In particular, we shall explore the elegant model of dynamic multithreaded algorithms, which are amenable to algorithmic design and analysis, as well as to efficient implementation in practice. Parallel computers—computers with multiple processing units—have become increasingly common, and they span a wide range of prices and performance. Rela- tively inexpensive desktop and laptop chip multiprocessors contain a single multi- core integrated-circuit chip that houses multiple processing “cores,” each of which is a full-fledged processor that can access a common memory. At an intermedi- ate price/performance point are clusters built from individual computers—often simple PC-class machines—with a dedicated network interconnecting them. The highest-priced machines are supercomputers, which often use a combination of custom architectures and custom networks to deliver the highest performance in terms of instructions executed per second. Multiprocessor computers have been around, in one form or another, for decades. Although the computing community settled on the random-access ma- chine model for serial computing early on in the history of computer science, no single model for parallel computing has gained as wide acceptance. A major rea- son is that vendors have not agreed on a single architectural model for parallel computers. For example, some parallel computers feature shared memory, where each processor can directly access any location of memory. Other parallel com- puters employ distributed memory, where each processor’s memory is private, and an explicit message must be sent between processors in order for one processor to access the memory of another. With the advent of multicore technology, however, every new laptop and desktop machine is now a shared-memory parallel computer, Chapter 27 Multithreaded Algorithms 773 and the trend appears to be toward shared-memory multiprocessing. Although time will tell, that is the approach we shall take in this chapter. One common means of programming chip multiprocessors and other shared- memory parallel computers is by using static threading, which provides a software abstraction of “virtual processors,” or threads, sharing a common memory. Each thread maintains an associated program counter and can execute code indepen- dently of the other threads. The operating system loads a thread onto a processor for execution and switches it out when another thread needs to run. Although the operating system allows programmers to create and destroy threads, these opera- tions are comparatively slow. Thus, for most applications, threads persist for the duration of a computation, which is why we call them “static.” Unfortunately, programming a shared-memory parallel computer directly using static threads is difficult and error-prone. One reason is that dynamically parti- tioning the work among the threads so that each thread receives approximately the same load turns out to be a complicated undertaking. For any but the sim- plest of applications, the programmer must use complex communication protocols to implement a scheduler to load-balance the work. This state of affairs has led toward the creation of concurrency platforms, which provide a layer of software that coordinates, schedules, and manages the parallel-computing resources. Some concurrency platforms are built as runtime libraries, but others provide full-fledged parallel languages with compiler and runtime support. Dynamic multithreaded programming One important class of concurrency platform is dynamic multithreading, which is the model we shall adopt in this chapter. Dynamic multithreading allows program- mers to specify parallelism in applications without worrying about communication protocols, load balancing, and other vagaries of static-thread programming. The concurrency platform contains a scheduler, which load-balances the computation automatically, thereby greatly simplifying the programmer’s chore. Although the functionality of dynamic-multithreading environments is still evolving, almost all support two features: nested parallelism and parallel loops. Nested parallelism allows a subroutine to be “spawned,” allowing the caller to proceed while the spawned subroutine is computing its result. A parallel loop is like an ordinary for loop, except that the iterations of the loop can execute concurrently. These two features form the basis of the model for dynamic multithreading that we shall study in this chapter. A key aspect of this model is that the programmer needs to specify only the logical parallelism within a computation, and the threads within the underlying concurrency platform schedule and load-balance the compu- tation among themselves. We shall investigate multithreaded algorithms written for 774 Chapter 27 Multithreaded Algorithms this model, as well how the underlying concurrency platform can schedule compu- tations efficiently. Our model for dynamic multithreading offers several important advantages: � It is a simple extension of our serial programming model. We can describe a multithreaded algorithm by adding to our pseudocode just three “concurrency” keywords: parallel, spawn, and sync. Moreover, if we delete these concur- rency keywords from the multithreaded pseudocode, the resulting text is serial pseudocode for the same problem, which we call the “serialization” of the mul- tithreaded algorithm. � It provides a theoretically clean way to quantify parallelism based on the no- tions of “work” and “span.” � Many multithreaded algorithms involving nested parallelism follow naturally from the divide-and-conquer paradigm. Moreover, just as serial divide-and- conquer algorithms lend themselves to analysis by solving recurrences, so do multithreaded algorithms. � The model is faithful to how parallel-computing practice is evolving. A grow- ing number of concurrency platforms support one variant or another of dynamic multithreading, including Cilk [51, 118], Cilk++ [72], OpenMP [60], Task Par- allel Library [230], and Threading Building Blocks [292]. Section 27.1 introduces the dynamic multithreading model and presents the met- rics of work, span, and parallelism, which we shall use to analyze multithreaded algorithms. Section 27.2 investigates how to multiply matrices with multithread- ing, and Section 27.3 tackles the tougher problem of multithreading merge sort. 27.1 The basics of dynamic multithreading We shall begin our exploration of dynamic multithreading using the example of computing Fibonacci numbers recursively. Recall that the Fibonacci numbers are defined by recurrence (3.22): F0 D 0 ; F1 D 1 ; Fi D Fi�1 C Fi�2 for i � 2 : Here is a simple, recursive, serial algorithm to compute the nth Fibonacci number: 27.1 The basics of dynamic multithreading 775 FIB.0/ FIB.0/FIB.0/FIB.0/ FIB.0/ FIB.1/FIB.1/ FIB.1/ FIB.1/ FIB.1/FIB.1/FIB.1/ FIB.1/ FIB.2/ FIB.2/FIB.2/FIB.2/ FIB.2/ FIB.3/FIB.3/ FIB.3/ FIB.4/ FIB.4/ FIB.5/ FIB.6/ Figure 27.1 The tree of recursive procedure instances when computing FIB.6/. Each instance of FIB with the same argument does the same work to produce the same result, providing an inefficient but interesting way to compute Fibonacci numbers. FIB.n/ 1 if n � 1 2 return n 3 else x D FIB.n � 1/ 4 y D FIB.n � 2/ 5 return x C y You would not really want to compute large Fibonacci numbers this way, be- cause this computation does much repeated work. Figure 27.1 shows the tree of recursive procedure instances that are created when computing F6. For example, a call to FIB.6/ recursively calls FIB.5/ and then FIB.4/. But, the call to FIB.5/ also results in a call to FIB.4/. Both instances of FIB.4/ return the same result (F4 D 3). Since the FIB procedure does not memoize, the second call to FIB.4/ replicates the work that the first call performs. Let T .n/ denote the running time of FIB.n/. Since FIB.n/ contains two recur- sive calls plus a constant amount of extra work, we obtain the recurrence T .n/ D T .n � 1/C T .n � 2/C‚.1/ : This recurrence has solution T .n/ D ‚.Fn/, which we can show using the substi- tution method. For an inductive hypothesis, assume that T .n/ � aFn � b, where a > 1 and b > 0 are constants. Substituting, we obtain 776 Chapter 27 Multithreaded Algorithms T .n/ � .aFn�1 � b/C .aFn�2 � b/C‚.1/ D a.Fn�1 C Fn�2/ � 2b C‚.1/ D aFn � b � .b �‚.1// � aFn � b if we choose b large enough to dominate the constant in the ‚.1/. We can then choose a large enough to satisfy the initial condition. The analytical bound T .n/ D ‚.�n/ ; (27.1) where � D .1 C p 5/=2 is the golden ratio, now follows from equation (3.25). Since Fn grows exponentially in n, this procedure is a particularly slow way to compute Fibonacci numbers. (See Problem 31-3 for much faster ways.) Although the FIB procedure is a poor way to compute Fibonacci numbers, it makes a good example for illustrating key concepts in the analysis of multithreaded algorithms. Observe that within FIB.n/, the two recursive calls in lines 3 and 4 to FIB.n� 1/ and FIB.n� 2/, respectively, are independent of each other: they could be called in either order, and the computation performed by one in no way affects the other. Therefore, the two recursive calls can run in parallel. We augment our pseudocode to indicate parallelism by adding the concurrency keywords spawn and sync. Here is how we can rewrite the FIB procedure to use dynamic multithreading: P-FIB.n/ 1 if n � 1 2 return n 3 else x D spawn P-FIB.n � 1/ 4 y D P-FIB.n � 2/ 5 sync 6 return x C y Notice that if we delete the concurrency keywords spawn and sync from P-FIB, the resulting pseudocode text is identical to FIB (other than renaming the procedure in the header and in the two recursive calls). We define the serialization of a mul- tithreaded algorithm to be the serial algorithm that results from deleting the multi- threaded keywords: spawn, sync, and when we examine parallel loops, parallel. Indeed, our multithreaded pseudocode has the nice property that a serialization is always ordinary serial pseudocode to solve the same problem. Nested parallelism occurs when the keyword spawn precedes a procedure call, as in line 3. The semantics of a spawn differs from an ordinary procedure call in that the procedure instance that executes the spawn—the parent—may continue to execute in parallel with the spawned subroutine—its child—instead of waiting 27.1 The basics of dynamic multithreading 777 for the child to complete, as would normally happen in a serial execution. In this case, while the spawned child is computing P-FIB.n � 1/, the parent may go on to compute P-FIB.n � 2/ in line 4 in parallel with the spawned child. Since the P-FIB procedure is recursive, these two subroutine calls themselves create nested parallelism, as do their children, thereby creating a potentially vast tree of subcom- putations, all executing in parallel. The keyword spawn does not say, however, that a procedure must execute con- currently with its spawned children, only that it may. The concurrency keywords express the logical parallelism of the computation, indicating which parts of the computation may proceed in parallel. At runtime, it is up to a scheduler to deter- mine which subcomputations actually run concurrently by assigning them to avail- able processors as the computation unfolds. We shall discuss the theory behind schedulers shortly. A procedure cannot safely use the values returned by its spawned children until after it executes a sync statement, as in line 5. The keyword sync indicates that the procedure must wait as necessary for all its spawned children to complete be- fore proceeding to the statement after the sync. In the P-FIB procedure, a sync is required before the return statement in line 6 to avoid the anomaly that would occur if x and y were summed before x was computed. In addition to explicit synchronization provided by the sync statement, every procedure executes a sync implicitly before it returns, thus ensuring that all its children terminate before it does. A model for multithreaded execution It helps to think of a multithreaded computation—the set of runtime instruc- tions executed by a processor on behalf of a multithreaded program—as a directed acyclic graph G D .V;E/, called a computation dag. As an example, Figure 27.2 shows the computation dag that results from computing P-FIB.4/. Conceptually, the vertices in V are instructions, and the edges in E represent dependencies be- tween instructions, where .u; �/ 2 E means that instruction u must execute before instruction �. For convenience, however, if a chain of instructions contains no parallel control (no spawn, sync, or return from a spawn—via either an explicit return statement or the return that happens implicitly upon reaching the end of a procedure), we may group them into a single strand, each of which represents one or more instructions. Instructions involving parallel control are not included in strands, but are represented in the structure of the dag. For example, if a strand has two successors, one of them must have been spawned, and a strand with mul- tiple predecessors indicates the predecessors joined because of a sync statement. Thus, in the general case, the set V forms the set of strands, and the set E of di- rected edges represents dependencies between strands induced by parallel control. 778 Chapter 27 Multithreaded Algorithms P-FIB(1) P-FIB(0) P-FIB(3) P-FIB(4) P-FIB(1) P-FIB(1) P-FIB(0) P-FIB(2) P-FIB(2) Figure 27.2 A directed acyclic graph representing the computation of P-FIB.4/. Each circle rep- resents one strand, with black circles representing either base cases or the part of the procedure (instance) up to the spawn of P-FIB.n � 1/ in line 3, shaded circles representing the part of the pro- cedure that calls P-FIB.n� 2/ in line 4 up to the sync in line 5, where it suspends until the spawn of P-FIB.n � 1/ returns, and white circles representing the part of the procedure after the sync where it sums x and y up to the point where it returns the result. Each group of strands belonging to the same procedure is surrounded by a rounded rectangle, lightly shaded for spawned procedures and heavily shaded for called procedures. Spawn edges and call edges point downward, continuation edges point horizontally to the right, and return edges point upward. Assuming that each strand takes unit time, the work equals 17 time units, since there are 17 strands, and the span is 8 time units, since the critical path—shown with shaded edges—contains 8 strands. If G has a directed path from strand u to strand �, we say that the two strands are (logically) in series. Otherwise, strands u and � are (logically) in parallel. We can picture a multithreaded computation as a dag of strands embedded in a tree of procedure instances. For example, Figure 27.1 shows the tree of procedure instances for P-FIB.6/ without the detailed structure showing strands. Figure 27.2 zooms in on a section of that tree, showing the strands that constitute each proce- dure. All directed edges connecting strands run either within a procedure or along undirected edges in the procedure tree. We can classify the edges of a computation dag to indicate the kind of dependen- cies between the various strands. A continuation edge .u; u0/, drawn horizontally in Figure 27.2, connects a strand u to its successor u0 within the same procedure instance. When a strand u spawns a strand �, the dag contains a spawn edge .u; �/, which points downward in the figure. Call edges, representing normal procedure calls, also point downward. Strand u spawning strand � differs from u calling � in that a spawn induces a horizontal continuation edge from u to the strand u0 fol- 27.1 The basics of dynamic multithreading 779 lowing u in its procedure, indicating that u0 is free to execute at the same time as �, whereas a call induces no such edge. When a strand u returns to its calling procedure and x is the strand immediately following the next sync in the calling procedure, the computation dag contains return edge .u; x/, which points upward. A computation starts with a single initial strand—the black vertex in the procedure labeled P-FIB.4/ in Figure 27.2—and ends with a single final strand—the white vertex in the procedure labeled P-FIB.4/. We shall study the execution of multithreaded algorithms on an ideal paral- lel computer, which consists of a set of processors and a sequentially consistent shared memory. Sequential consistency means that the shared memory, which may in reality be performing many loads and stores from the processors at the same time, produces the same results as if at each step, exactly one instruction from one of the processors is executed. That is, the memory behaves as if the instructions were executed sequentially according to some global linear order that preserves the individual orders in which each processor issues its own instructions. For dynamic multithreaded computations, which are scheduled onto processors automatically by the concurrency platform, the shared memory behaves as if the multithreaded computation’s instructions were interleaved to produce a linear order that preserves the partial order of the computation dag. Depending on scheduling, the ordering could differ from one run of the program to another, but the behavior of any exe- cution can be understood by assuming that the instructions are executed in some linear order consistent with the computation dag. In addition to making assumptions about semantics, the ideal-parallel-computer model makes some performance assumptions. Specifically, it assumes that each processor in the machine has equal computing power, and it ignores the cost of scheduling. Although this last assumption may sound optimistic, it turns out that for algorithms with sufficient “parallelism” (a term we shall define precisely in a moment), the overhead of scheduling is generally minimal in practice. Performance measures We can gauge the theoretical efficiency of a multithreaded algorithm by using two metrics: “work” and “span.” The work of a multithreaded computation is the total time to execute the entire computation on one processor. In other words, the work is the sum of the times taken by each of the strands. For a computation dag in which each strand takes unit time, the work is just the number of vertices in the dag. The span is the longest time to execute the strands along any path in the dag. Again, for a dag in which each strand takes unit time, the span equals the number of vertices on a longest or critical path in the dag. (Recall from Section 24.2 that we can find a critical path in a dag G D .V;E/ in ‚.V C E/ time.) For example, the computation dag of Figure 27.2 has 17 vertices in all and 8 vertices on its critical 780 Chapter 27 Multithreaded Algorithms path, so that if each strand takes unit time, its work is 17 time units and its span is 8 time units. The actual running time of a multithreaded computation depends not only on its work and its span, but also on how many processors are available and how the scheduler allocates strands to processors. To denote the running time of a multithreaded computation on P processors, we shall subscript byP . For example, we might denote the running time of an algorithm on P processors by TP . The work is the runnin