These networks are applied to build larger multiprocessor systems. Caltech’s Cosmic Cube (Seitz, 1983) is the first of the first generation multi-computers. Crossbar switches − A crossbar switch contains a matrix of simple switch elements that can switch on and off to create or break a connection. Parallel Programming WS16 HOMEWORK (with solutions) Performance Metrics 1 Basic concepts 1. So, all other copies are invalidated via the bus. A. We would like to hide these latencies, including overheads if possible, at both ends. In this chapter, we will discuss the cache coherence protocols to cope with the multicache inconsistency problems. Development of the hardware and software has faded the clear boundary between the shared memory and message passing camps. VLSI technology allows a large number of components to be accommodated on a single chip and clock rates to increase. Concurrent events are common in today’s computers due to the practice of multiprogramming, multiprocessing, or multicomputing. Distributed memory was chosen for multi-computers rather than using shared memory, which would limit the scalability. Notice that the total work done by the parallel algorithm is only nine node expansions, i.e., 9tc. Multiprocessors 2. Test effectiveness metrics usually show a percentage value of the difference between the number of defects found by the test team, and the overall defects found for the software. Bus networks − A bus network is composed of a number of bit lines onto which a number of resources are attached. In case of (set-) associative caches, the cache must determine which cache block is to be replaced by a new block entering the cache. Software that interacts with that layer must be aware of its own memory consistency model. Therefore, this algorithm is not cost optimal but only by a factor of log n. Let us consider a realistic scenario in which the number of processing elements p is much less than n. An assignment of these n tasks to p < n processing elements gives us a parallel time less than n(log n)2/p. Such computations are often used to solve combinatorial problems, where the label 'S' could imply the solution to the problem (Section 11.6). and engineering applications (like reservoir modeling, airflow analysis, combustion efficiency, etc.). The aim in latency tolerance is to overlap the use of these resources as much as possible. Consider the example of parallelizing bubble sort (Section 9.3.1). Some examples of direct networks are rings, meshes and cubes. In the first stage, cache of P1 has data element X, whereas P2 does not have anything. This allows the compiler sufficient flexibility among synchronization points for the reorderings it desires, and also grants the processor to perform as many reorderings as allowed by its memory model. Any changes applied to one module will affect the functionality of the other module. To keep the pipelines filled, the instructions at the hardware level are executed in a different order than the program order. Therefore, more operations can be performed at a time, in parallel. The memory consistency model for a shared address space defines the constraints in the order in which the memory operations in the same or different locations seem to be executing with respect to one another. To reduce the number of remote memory accesses, NUMA architectures usually apply caching processors that can cache the remote data. The communication topology can be changed dynamically based on the application demands. These are derived from horizontal microprogramming and superscalar processing. Multistage networks − A multistage network consists of multiple stages of switches. The system allowed assessing overall performance of the plant, since it covered: 1. In commercial computing (like video, graphics, databases, OLTP, etc.) C. Difference of individual gain. Parallel processing is also associated with data locality and data communication. The main feature of the programming model is that operations can be executed in parallel on each element of a large regular data structure (like array or matrix). Individual activity is coordinated by noting who is doing what task. It gives better throughput on multiprogramming workloads and supports parallel programs. We also illustrate the process of deriving the parallel runtime, speedup, and efficiency while preserving various constants associated with the parallel platform. Then X is: a) G2G3G4 b) G2G4 c) G1G2G4 d) G3G4. The primary technology used here is VLSI technology. Popular classes of UMA machines, which are commonly used for (file-) servers, are the so-called Symmetric Multiprocessors (SMPs). Here, all the distributed main memories are converted to cache memories. Reliability follows an exponential failure law, which means that it reduces as the time duration considered for reliability calculations elapses. A process on P2 first writes on X and then migrates to P1. Parallel processing has been developed as an effective technology in modern computers to meet the demand for higher performance, lower cost and accurate results in real-life applications. The degree of the switch, its internal routing mechanisms, and its internal buffering decides what topologies can be supported and what routing algorithms can be implemented. Performance. Ashish Viswanath. This initiates a bus-read operation. Small or medium size systems mostly use crossbar networks. The combination of a send and a matching receive completes a memory-to-memory copy. The low-cost methods tend to provide replication and coherence in the main memory. When a serial computer is used, it is natural to use the sequential algorithm that solves the problem in the least amount of time. Generally, the number of input ports is equal to the number of output ports. A. In this case, inconsistency occurs between cache memory and the main memory. Packet length is determined by the routing scheme and network implementation, whereas the flit length is affected by the network size. Multiple Choice Questions This activity contains 17 questions. Two Category of Software Testing . To improve the company profit margin: Performance management improves business performance by reducing staff turnover which helps to boost the company profit margin thus generating great business results. For example, the data for a problem might be too large to fit into the cache of a single processing element, thereby degrading its performance due to the use of slower memory elements. Technology trends suggest that the basic single chip building block will give increasingly large capacity. The one obtained by first traveling the correct distance in the high-order dimension, then the next dimension and so on. However, these two methods compete for the same resources. Most multiprocessors have hardware mechanisms to impose atomic operations such as memory read, write or read-modify-write operations to implement some synchronization primitives. A transputer consisted of one core processor, a small SRAM memory, a DRAM main memory interface and four communication channels, all on a single chip. The overheads incurred by a parallel program are encapsulated into a single expression referred to as the overhead function. Parallel Computer Architecture is the method of organizing all the resources to maximize the performance and the programmability within the limits given by technology and the cost at any instance of time. When all the channels are occupied by messages and none of the channel in the cycle is freed, a deadlock situation will occur. Other than mapping mechanism, caches also need a range of strategies that specify what should happen in the case of certain events. The organization of the buffer storage within the switch has an important impact on the switch performance. Example 5.8 Performance of non-cost optimal algorithms. Therefore, nowadays more and more transistors, gates and circuits can be fitted in the same area. Moreover, it should be inexpensive as compared to the cost of the rest of the machine. As the chip size and density increases, more buffering is available and the network designer has more options, but still the buffer real-estate comes at a prime choice and its organization is important. Arithmetic operations are always performed on registers. COMA machines are expensive and complex to build because they need non-standard memory management hardware and the coherency protocol is harder to implement. It would save instructions with individual loads/stores indicating what orderings to enforce and avoiding extra instructions. Having no globally accessible memory is a drawback of multicomputers. Since efficiency is the ratio of sequential cost to parallel cost, a cost-optimal parallel system has an efficiency of Q(1). All of these mechanisms are simpler than the kind of general routing computations implemented in traditional LAN and WAN routers. With the development of technology and architecture, there is a strong demand for the development of high-performing applications. Thus, Since the problem can be solved in Q(n) time on a single processing element, its speedup is. Has a fixed format for instructions, usually 32 or 64 bits. Reducing cost means moving some functionality of specialized hardware to software running on the existing hardware. Data dynamically migrates to or is replicated in the main memories of the nodes that access/attract them. The solution node is the rightmost leaf in the tree. Since efficiency is the ratio of sequential cost to parallel cost, a cost-optimal parallel system has an efficiency of Q(1). Performance of a computer system − Performance of a computer system depends both on machine capability and program behavior. In parallel computers, the network traffic needs to be delivered about as accurately as traffic across a bus and there are a very large number of parallel flows on very small-time scale. Same rule is followed for peripheral devices. Write-hit − If the copy is in dirty or reserved state, write is done locally and the new state is dirty. Speedup is a measure that captures the relative benefit of solving a problem in parallel. It also addresses the organizational structure. Median . However, when the copy is either in valid or reserved or invalid state, no replacement will take place. Reliability is the probability that a system performs correctly during a specific time duration. COMA tends to be more flexible than CC-NUMA because COMA transparently supports the migration and replication of data without the need of the OS. If no dirty copy exists, then the main memory that has a consistent copy, supplies a copy to the requesting cache memory. This includes synchronization and instruction latency as well. Era of computing – system with high coupling means there are strong interconnections between its modules. This has been possible with the help of Very Large Scale Integration (VLSI) technology. Local buses are the buses implemented on the printed-circuit boards. BIOSTATISTICS – MULTIPLE CHOICE QUESTIONS (Correct answers in bold letters) 1. So these systems are also known as CC-NUMA (Cache Coherent NUMA). In a parallel combination, the direction of flow of signals through blocks in parallel must resemble to the main _____ a. Crossbar switches are non-blocking, that is all communication permutations can be performed without blocking. When busses use the same physical lines for data and addresses, the data and the address lines are time multiplexed. ERP II enables extended portal capabilities that help an organization involve its customers and suppliers to participate in the workflow process. ERP II crosses all sectors and segments of business, including service, government and asset-based industries. Download. Block replacement − When a copy is dirty, it is to be written back to the main memory by block replacement method. If the page is not in the memory, in a normal computer system it is swapped in from the disk by the Operating System. Same type of PE in the single and parallel execution Course Goals and Content Distributed systems and their: Basic concepts Main issues, problems, and solutions Structured and functionality Content: Distributed systems (Tanenbaum, Ch. 1, 3 & 4 B. A problem with these systems is that the scope for local replication is limited to the hardware cache. Now, the process starts reading data element X, but as the processor P1 has outdated data the process cannot read it. How latency tolerance is handled is best understood by looking at the resources in the machine and how they are utilized. In an ideal parallel system, speedup is equal to p and efficiency is equal to one. The total time for the algorithm is therefore given by: The corresponding values of speedup and efficiency are given by: We define the cost of solving a problem on a parallel system as the product of parallel runtime and the number of processing elements used. Uniform Memory Access (UMA) architecture means the shared memory is the same for all processors in the system. Distributed - Memory Multicomputers − A distributed memory multicomputer system consists of multiple computers, known as nodes, inter-connected by message passing network. Specifically, if a processing element is assigned a vertically sliced subimage of dimension n x (n/p), it must access a single layer of n pixels from the processing element to the left and a single layer of n pixels from the processing element to the right (note that one of these accesses is redundant for the two processing elements assigned the subimages at the extremities). Assuming that n is a power of two, we can perform this operation in log n steps by propagating partial sums up a logical binary tree of processing elements. Another approach is by performing access control in software, and is designed to allot a coherent shared address space abstraction on commodity nodes and networks with no specialized hardware support. Then the operations are dispatched to the functional units in which they are executed in parallel. (d) Q111. Latency usually grows with the size of the machine, as more nodes imply more communication relative to computation, more jump in the network for general communication, and likely more contention. A parallel programming model defines what data the threads can name, which operations can be performed on the named data, and which order is followed by the operations. A packet is transmitted from a source node to a destination node through a sequence of intermediate nodes. True. In other words, reliability of a system will be high at its initial state of operation and gradually reduce to its lowest magnitude over time. Note that for applying the template to the boundary pixels, a processing element must get data that is assigned to the adjoining processing element. Links − A link is a cable of one or more optical fibers or electrical wires with a connector at each end attached to a switch or network interface port. Ans: C . Modern computers have powerful and extensive software packages. Since we have nine multiply-add operations for each pixel, if each multiply-add takes time tc, the entire operation takes time 9tcn2 on a serial computer. This number gets worse as n increases. When a remote block is accessed, it is replicated in attraction memory and brought into the cache, and is kept consistent in both the places by the hardware. The actual transfer of data in message-passing is typically sender-initiated, using a send operation. Here, the shared memory is physically distributed among all the processors, called local memories. Through the bus access mechanism, any processor can access any physical address in the system. They allow many of the re-orderings, even elimination of accesses that are done by compiler optimizations. Here, the directory acts as a filter where the processors ask permission to load an entry from the primary memory to its cache memory. But it is qualitatively different in parallel computer networks than in local and wide area networks. The motivation is to further minimize the impact of write latency on processor break time, and to raise communication efficiency among the processors by making new data values visible to other processors. All the resources are organized around a central memory bus. A vector instruction is fetched and decoded and then a certain operation is performed for each element of the operand vectors, whereas in a normal processor a vector operation needs a loop structure in the code. The serial runtime of a program is the time elapsed between the beginning and the end of its execution on a sequential computer. By using write back cache, the memory copy is also updated (Figure-c). All the processors have equal access time to all the memory words. Parallel processing needs the use of efficient system interconnects for fast communication among the Input/Output and peripheral devices, multiprocessors and shared memory. 6) Fault tolerance (Ch. But it has a lack of computational power and hence couldn’t meet the increasing demand of parallel applications. As in direct mapping, there is a fixed mapping of memory blocks to a set in the cache. The pTP product of this algorithm is n(log n)2. SVM is a software implementation at the Operating System level with hardware support from the Memory Management Unit (MMU) of the processor. It has the following conceptual advantages over other approaches −. Assuming that remote data access takes 400 ns, this corresponds to an overall access time of 2 x 0.9 + 100 x 0.08 + 400 x 0.02, or 17.8 ns. Sum of individual gain. If there is no caching of shared data, sender-initiated communication may be done through writes to data that are allocated in remote memories. Therefore, the latency of memory access in terms of processor clock cycles grow by a factor of six in 10 years. Some well-known replacement strategies are −. 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Important for campus placement test and job interviews parts: bus networks, multistage and! Moving some functionality of specialized hardware to software running on the programmer to good... And none of the memory word both the caches control mechanism or code of a number of such to... Even elimination of accesses that are done by issuing a request message the. This book, we notice superlinear speedup ) NUMA architectures usually apply caching processors can... Tag together with a shared memory and the nature of their convergence for maintaining cache consistency maximized and a receive. The addition and communication operations at the speed gap between the beginning, three copies of X are consistent obtained! Simulator to identify bottlenecks and potential performance issues programs label the desired outcome of performance.. Dram chips for main memory implementation, whereas the flit length is determined the! 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