Mesh Multicomputers Research Articles

Communication-Load Impact on the Performance of Processor Allocation Strategies in 2-D Mesh Multicomputer Systems

A number of processor allocation strategies have been proposed in literature. A key performance factor that can highlight the difference between these strategies is the amount of communication conducted between the parallel jobs to be allocated. This paper aims to identifying how the density and pattern of communication can affect the performance of these strategies. Compared to the work already presented in literature, we examined a wider range of communication patterns. Other works consider only two types of communication patterns; those are the one-to-all and all-to-all patterns. This paper used the C language to implement the different allocation strategies, and then combined them with the ProcSimity simulation tool. The processor-allocation strategies are examined under the First-Come-First-Serve scheduling strategy. Results show that communication pattern and load are effective factors that have a significant impact on the performance of the processor allocation strategy used.

A New Compacting Non-Contiguous Processor Allocation Algorithm for 2D Mesh Multicomputers

In non-contiguous allocation, a job request can be split into smaller parts that are allocated possibly non-adjacent free sub-meshes rather than always waiting until a single sub-mesh of the requested size and shape is available. Lifting the contiguity condition is expected to reduce processor fragmentation and increase system utilization. However, the distances traversed by messages can be long, and as a result the communication overhead, especially contention, is likely to increase. The extra communication overhead depends on how the allocation request is partitioned and assigned to free sub-meshes. In this paper, a new non-contiguous processor allocation strategy, referred to as Compacting Non-Contiguous Processor Allocation Strategy (CNCPA), is suggested for the 2D mesh multicomputers. In the proposed strategy, a job is compacted into free locations. The selection of the free locations has for goal leaving large free sub-meshes in the system. To evaluate the performance improvement achieved by the proposed strategy and compare it against well-known existing non-contiguous allocation strategies, the authors conducted extensive simulation experiments. The results show that the proposed strategy can improve performance in terms of job turnaround times and system utilization.

The Effect of Real Workloads and Synthetic Workloads on the Performance of Job Scheduling for Non-Contiguous Allocation in 2D Mesh Multicomputers

The performance of non-contiguous allocation has been traditionally carried out by means of simulations based on synthetic workloads, and also it can be significantly affected by the job scheduling strategy used for determining the order in which jobs are selected for execution. To validate the performance of the non-contiguous allocation algorithms, there has been a need to evaluate the algorithm's performance based on a real workload trace. In this paper, the performance of the well-known Greedy Available Busy List (GABL) non-contiguous allocation strategy for 2D mesh-connected multicomputers is revisited considering several important job scheduling strategies based on a real workload trace, and the results are compared to those obtained from using a synthetic workload. The scheduling strategies used are the First-Come-First-Served (FCFS), Out-of-Order (OO), and Window-Based job scheduling strategies. These strategies have been selected because they are common and they have been used in related works (Ababneh & Bani-Mohammad, 2011). Extensive simulation results based on synthetic and real workload models indicate that the Window-Based job scheduling strategy can improve both overall system performance and fairness (i.e., maximum job waiting delays) by adopting a large job scheduling window. Moreover, the relative performance merits of the scheduling strategies when a real workload trace is used are in general compatible with those obtained when a synthetic workload is used.

On the performance of non-contiguous allocation for common communication patterns in 2D mesh-connected multicomputers

The communication pattern used by applications can have a major influence on the performance of non-contiguous processor allocation in multicomputers. In this paper, the performance of well-known non-contiguous allocation strategies for 2D mesh multicomputers is re-visited considering several important communication patterns. These are the Near Neighbour, Ring, Divide and Conquer Binomial Tree (DQBT), Fast Fourier Transform (FFT), and Random communication patterns. The allocation strategies investigated are the Greedy Available Busy List (GABL), Multiple Buddy Strategy (MBS), Adaptive Non-contiguous Allocation (ANCA), and Paging(0). They are compared using detailed flit-level simulations. The results show that GABL is overall superior to the remaining non-contiguous allocation strategies. It produces superior average turnaround times and mean system utilization.

A new window-based job scheduling scheme for 2D mesh multicomputers

Allocating submeshes to jobs in mesh-connected multicomputers in a FCFS fashion can lead to poor system performance (e.g., long job waiting delays) because the job at the head of the waiting queue can prevent the allocation of free submeshes to other waiting jobs with smaller submesh requirements. However, serving jobs aggressively out-of-order can lead to excessive waiting delays for jobs with large allocation requests. In this paper, we propose a scheduling scheme that uses a window of consecutive jobs from which it selects jobs for allocation and execution. This window starts with the current oldest waiting job and corresponds to the lookahead of the scheduler. The performance of the proposed window-based scheme has been compared to that of FCFS and other previous job scheduling schemes. Extensive simulation results based on synthetic workloads and real workload traces indicate that the new scheduling strategy exhibits good performance when the scheduling window size is large. In particular, it is substantially superior to FCFS in terms of system utilization, average job turnaround times, and maximum waiting delays under medium to heavy system loads. Also, it is superior to aggressive out-of-order scheduling in terms of maximum job waiting delays. Window-based job scheduling can improve both overall system performance and fairness (i.e., maximum job waiting delays) by adopting large lookahead job scheduling windows.

Task migration in three-dimensional meshes

As a result of the emerging use of mesh-based multicomputers (and recently mesh-based multiprocessor systems-on-chip), issues related to processor management have attracted much attention. In a mesh-based multiprocessor, after repeated submesh allocations and de-allocations, the system network may be fragmented, i.e. there might be unallocated nodes in the network. As a result, in a system with contiguous processor allocation, no new tasks can start running due to the lack of enough free adjacent processors to form a suitable submesh. Although there might be enough free processors available, they remain idle until the allocator can find a set of adjacent free nodes forming a submesh to be used for the new task. This can lead to low system performance. Task migration was introduced as a solution to this problem through migration of tasks running on some submeshes to other free areas in order to reduce fragmentation by chaining the newly freed areas and disengaging nodes to form larger submeshes. In this paper, we propose a novel structured and formulated way to code task migration, which is helpful for congestion detection in different steps of task migration algorithms. Moreover, considering the fact that the 3D mesh-based multicomputers are now very popular, a new task migration algorithm in 3D meshes is proposed. We also address the special case of the 2D migration in a 3D mesh multicomputer.

Fault Tolerance and Reliability of Mesh Multi-computer Networks � A Review

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On submesh allocation for 2D mesh multicomputers using the free-list approach: Global placement schemes

Two global placement schemes for contiguous processor allocation in two-dimensional mesh-connected multicomputers are proposed in this paper. The first scheme gives preference to allocating a free peripheral submesh that has the largest number of mesh-boundary processors. This peripheral placement has for goal producing large leftover free submeshes, which can improve system performance. Another characteristic of this scheme is that it reduces the search space by halting the search process when a large-enough multicomputer corner submesh is found. The second proposed scheme considers allocation in the corners of all large-enough free submeshes and allocates a submesh that has the maximum number of allocated neighbors and multicomputer peripheral nodes. Using extensive simulations, we evaluated the proposed schemes and compared them with previous promising schemes. The simulation results show that the peripheral placement scheme produces the best average turnaround times, and its measured allocation and de-allocation times are smaller than those of the previous schemes. The second proposed scheme ranks overall second in terms of turnaround times, however it is last in terms of efficiency.

Comparative evaluation of contiguous allocation strategies on 3D mesh multicomputers

The performance of contiguous allocation strategies can be significantly affected by the type of the distribution adopted for job execution times. In this paper, the performance of the existing contiguous allocation strategies for 3D mesh multicomputers is re-visited in the context of heavy-tailed distributions (e.g., a Bounded Pareto distribution). The strategies are evaluated and compared using simulation experiments for both First-Come-First-Served (FCFS) and Shortest-Service-Demand (SSD) scheduling strategies under a variety of system loads and system sizes. The results show that the performance of the allocation strategies degrades considerably when job execution times follow a heavy-tailed distribution. Moreover, SSD copes much better than FCFS scheduling strategy in the presence of heavy-tailed job execution times. The results also reveal that allocation strategies that employ a list of allocated sub-meshes for both allocation and de-allocation exhibit low allocation overhead, and maintain good system performance in terms of average turnaround time and mean system utilization.

An accurate performance model of fully adaptive routing in wormhole-switched two-dimensional mesh multicomputers

Numerous analytical performance models have been proposed for deterministic wormhole-routed mesh networks while only a single model, to our best knowledge, has been suggested for fully adaptive wormhole routing in mesh interconnection networks. This paper proposes a new and accurate analytical performance model for fully adaptive wormhole-routed mesh networks. Simulation results show that unlike the previously proposed model which is only accurate in light traffic loads, this model can be used for the performance analysis of almost all traffic loads.

Task migration in all-port wormhole-routed 2D mesh multicomputers

In a mesh multicomputer, submeshes are allocated to perform jobs according to processor allocation schemes, with each task assigned to occupy processors of one submesh with an appropriate size. To assign regions for incoming tasks, task compaction is needed to produce a large contiguous free region. The overhead of task compaction relies mainly on designing an efficient task migration scheme. This paper investigates task migration schemes in 2D wormhole-routed mesh multicomputers with an all-port communication model. Two constraints are given between two submeshes for task migration. Two task migration schemes that follow one of the constraints in 2D mesh multicomputers are then developed. In addition, the proposed schemes are proven to be deadlock-free and congestion-free. Finally, performance analysis is adopted to compare the proposed task migration schemes.

Task migration in n-dimensional wormhole-routed mesh multicomputers

In a mesh multicomputer, performing jobs needs to schedule submeshes according to some processor allocation scheme. In order to assign the incoming jobs to a free submesh, a task compaction scheme is needed to generate a larger contiguous free region. The overhead of compaction depends on the efficiency of the task migration scheme. In this paper, two simple task migration schemes are first proposed in n-dimensional mesh multicomputers with supporting dimension-ordered wormhole routing in one-port communication model. Then, a hybrid scheme which combines advantages of the two schemes is discussed. Finally, we evaluate the performance of all of these proposed approaches.

Hypercube algorithms on mesh connected multicomputers

A new methodology named CALMANT (CC-cube Algorithms on Meshes and Tori) for mapping a type of algorithm that we call CC-cube algorithm onto multicomputers with hypercube, mesh, or torus interconnection topology is proposed. This methodology is suitable when the initial problem can be expressed as a set of processes that communicate through a hypercube topology (a CC-cube algorithm). There are many important algorithms that fit into the CC-cube type. CALMANT is based on three different techniques: (a) the standard embedding to assign the processes of the algorithm to the nodes of the mesh multicomputer; (b) the communication pipelining technique to increase the level of communication parallelism inherent in the CC-cube algorithms; and (c) optimal message-scheduling algorithms proposed in this work in order to avoid conflicts and minimizing in this way the communication time. Although CALMANT is proposed for multicomputers with different interconnection network topologies, the paper only focuses on the particular case of meshes.

Fault-tolerant tree-based multicasting in mesh multicomputers

We propose a fault-tolerant tree-based multicast algorithm for 2-dimensional (2-D) meshes based on the concept of the extended safety level which is a vector associated with each node to capture fault information in the neighborhood. In this approach each destination is reached through a minimum number of hops. In order to minimize the total number of traffic steps, three heuristic strategies are proposed. This approach can be easily implemented by pipelined circuit switching (PCS). A simulation study is conducted to measure the total number of traffic steps under different strategies. Our approach is the first attempt to address the fault-tolerant tree-based multicast problem in 2-D meshes based on limited global information with a simple model and succinct information.

Implementing the one-sided Jacobi method on a 2D/3D mesh multicomputer

The paper discusses the implementation of a parallel algorithm to compute the eigenvalues and eigenvectors of a real symmetric matrix on a mesh multicomputer. The algorithm uses the one-sided Jacobi method and a two-dimensional organization of the nodes. It is aimed at reducing the communication cost incurred by one-dimensional algorithms found in the literature. The performance of the proposed algorithm on a squared 2D/3D mesh multicomputer is assessed through simple analytical models of execution time. The models show that the performance improvement over one-dimensional algorithms can be very noticeable, specially for a large number of nodes.

YOMNA – An efficient deadlock-free multicast wormhole algorithm in 2-D mesh multicomputers

Abstract A mesh network is a popular architecture which has been implemented in many multicomputer systems. It is preferred because it offers useful edge connectivity and is partitioned into units that are still meshes. It is also scalable and has a number of features that make it particularly amenable to high-performance computing. The 2-D mesh topology has become increasingly important to multicomputer design because of its many desirable properties including scalability, low bandwidth and fixed degree of nodes . The essential pattern in new multicomputer generations is the multicast wormhole pattern, which corresponds to one-to-many communication in which one source sends the same message to multiple destination nodes. In wormhole routing, a message is decomposed into words or flits , and flits of one message may be spread out among several nodes. Deadlock in the interconnection network occurs when no message can advance towards its destination. Some deadlock -free routing algorithms for wormhole routing were proposed, but the network latency and the network traffic were not taken into consideration. An optimal message routing should achieve both minimum traffic and minimum latency for the communication patterns involved. Unfortunately, finding optimal message routing has been shown to be NP-hard for most common multicomputer topologies. In this paper, an efficient algorithm (YOMNA) is introduced to find a deadlock-free multicast wormhole routing in 2-D mesh parallel machines. YOMNA algorithm is a tree-based technique, in which the router simultaneously sends incoming flits on more than one outgoing channel. YOMNA algorithm is compared with the dual-path multicast routing, which is a path-based technique. YOMNA algorithm has proved to be deadlock free. The network latency and the network traffic are calculated for YOMNA algorithm and for the dual-path multicast routing. The results demonstrate that YOMNA algorithm outperformed the dual-path routing.

Task migration in 2D wormhole-routed mesh multicomputers

In this paper, two simple task migration schemes are first proposed for 2D mesh, multicomputers with supporting X-Y wormhole routing in the one-port communication model. We next propose a hybrid task migration scheme which attempts to minimize the total transmission latency. Finally, we compare all of our proposed task migration schemes using performance analysis.

Adaptive and deadlock-free routing for irregular faulty patterns in mesh multicomputer

Message routing achieves the internode communication in parallel computers. A reliable routing is supposed to be deadlock-free and fault-tolerant. While many routing algorithms are able to tolerate a large number of faults enclosed by rectangular faulty blocks, there is no existing algorithm that is capable of handling irregular faulty patterns for wormhole networks. In this paper, a two-staged adaptive and deadlock-free routing algorithm called "Routing for Irregular Faulty Patterns" (RIFP) is proposed. It can tolerate irregular faulty patterns by transmitting messages from sources or to destinations within faulty blocks via multiple "intermediate nodes." A method employed by RIFP is first introduced to generate intermediate nodes using the local failure information. By its aid, two communicating nodes can always exchange their data or intermediate results if there is at least one path between them. RIFP needs two virtual channels per physical link in meshes.

Job scheduling in mesh multicomputers

A new approach for dynamic job scheduling in mesh-connected multiprocessor systems, which supports a multiuser environment, is proposed in this paper. Our approach combines a submesh reservation policy with a priority-based scheduling policy to obtain high performance in terms of high throughput, high utilization, and low turn-around times for jobs. This high performance is achieved at the expense of scheduling jobs in a strictly fair, FCFS fashion; in fact, the algorithm is parameterized to allow trade-offs between performance and (short-term) POPS fairness. The proposed scheduler can be used with any submesh allocation policy. A fast and efficient implementation of the proposed scheduler has also been presented. The performance of the proposed scheme has been compared with the FCFS policy, the only existing scheduling strategy for meshes, to demonstrate the effectiveness of the proposed approach. Simulation results indicate that our scheduling strategy outperforms the FCFS policy significantly. Specifically, our strategy significantly reduces the average waiting delay of jobs over the FCFS policy. The fast implementation of the proposed scheduler results in low allocation and deallocation time overhead, as well as low space overhead.

On submesh allocation for mesh multicomputers: a best-fit allocation and a virtual submesh allocation for faulty meshes

The submesh allocation problem is to recognize and locate a free submesh that can accommodate a request for a submesh of a specified size. In this paper, we propose a new best-fit submesh allocation strategy for mesh-connected multiprocessor systems. The proposed strategy maintains and uses a free submesh list for an efficient allocation. For an allocation request, the strategy selects the best-fit submesh which causes the least amount of potential processor fragmentation. As many large free submeshes as possible are preserved for later allocations. For this purpose, we introduce a novel function quantifying the degree of potential fragmentation of submeshes. The proposed strategy has the capability of recognizing a complete submesh. We also propose an allocation strategy for faulty meshes which can maintain and allocate virtual submeshes derived from faulty submeshes. Extensive simulation is carried out to compare the proposed strategy with previous strategies. The proposed strategy has the best performance: a 6-50 percent improvement over the previous best strategy.