Browsing by Subject "Computational grids (Computer systems)"

Now showing 1 - 6 of 6

Open Access
A bipartite graph model for placement, scheduling and replication in data grids
(2012) Dal, Burcu
Data grids provide geographically distributed resources for applications that generate and utilize large data sets. However, there are some issues that hinder to ensure fast access to data and low turnaround time for the jobs in data grids. To address these issues, several data replication and job scheduling strategies have been introduced to offer high data availability, low bandwidth consumption, and reduced turnaround time for grid systems. Multiple copies of existing data are maintained at different locations via data replication. Data replication strategies are broadly categorized as static and dynamic. In static replication strategies, replication is performed during the system design, and replica decisions are generally based on a cost model that includes data access costs, bandwidth characteristics and storage constraints of the grid system. In dynamic replication strategies, the replication operation is managed at runtime so that the system adapts to the changes in user request patterns dynamically. Job scheduling strategies fall under two main categories: online mode and batch mode. The online mode scheduler assigns tasks to sites as soon as they arrive. In the batch mode, the complete set of jobs are taken into account and scheduled at the same time by using all the grid information. In this thesis, we propose a bipartite graph model for tasks and files in the grid system, and then we partition this graph to obtain a data placement and job scheduling strategy. The obtained parts are further refined in order to be assigned to grid sites by using a KL-based heuristic that takes the bandwidth and hop information between sites into account. Replication is achieved by replicating a certain amount of most accessed files chosen prior to the partitioning process. Experimental results indicate that the increase in the partitioning quality reflects positively on the mapping quality. Morever, it is observed that the communication cost is notably decreased when the data replication is applied. Hence, our results show that by replicating a small amount of data files and placing files onto sites using bipartite graph model, we can obtain performance improvement for scheduling jobs compared to no replication.
Open Access
Comparative evaluation of spectrum allocation policies for dynamic flexgrid optical networks
(2013) Yümer, Ramazan
A novel class-based first-fit spectrum allocation policy is proposed for dynamic Flexgrid optical networks. The effectiveness of the proposed policy is compared against the first-fit policy for single-hop and multi-hop scenarios. Event-based simulation technique is used for testing the spectrum allocation policies under both Fixed Routing and Fixed Alternate Routing algorithms with two shortest paths. Throughput is shown to be consistently improved under the proposed policy with gains of up to 15 % in certain scenarios.
Open Access
Exploiting replicated data for communication load balancing in image-space parallel direct volume rendering of unstructured grids
(2009) Okuyan, Erkan
The focus of this work is on parallel volume rendering applications in which renderings with different parameters are successively repeated over the same dataset. The only reason for inter-task interaction is the existence of data primitives that are inputs to several tasks. Both computational structure and expected task execution times may change during successive rendering instances. Change in computational structure means change in the data primitive requirements of tasks. Since the individual processors of a parallel system have a limited storage capacity, we can reserve a limited amount of storage for holding replicas at each processor. For the parallelization of a particular rendering instance, the remapping model should utilize the replication pattern of the previous rendering instance(s) for reducing the communication overhead due to the data replication requirement of the current rendering instance. We propose a two-phase model for solving this problem. The hypergraphpartitioning-based model proposed for the first phase aims to minimize the total message volume that will be incurred due to the replication/migration of input data while maintaining balance on computational and receive-volume loads of processors. The network-flow-based model proposed for the second phase aims to minimize the maximum message volume handled by processors via utilizing the flexibility in assigning send-communication tasks to processors, which is introduced by data replication. The validity of our proposed model is verified on image-space parallelization of a direct volume rendering algorithm.
Open Access
Iterative-improvement-based heuristics for adaptive scheduling of tasks sharing files on heterogeneous master-slave environments
(2004) Kaya, Kamer
Open Access
Joint path and resource selection for OBS grids with adaptive offset based QOS mechanism
(2007) Köseoğlu, Mehmet
It is predicted that grid computing will be available for consumers performing their daily computational needs with the deployment of high bandwidth optical networks. Optical burst switching is a suitable switching technology for this kind of consumer grid networks because of its bandwidth granularity. However, high loss rates inherent in OBS has to be addressed to establish a reliable transmission infrastructure. In this thesis, we propose mechanisms to reduce loss rates in an OBS grid scenario by using network-aware resource selection and adaptive offset determination. We first propose a congestion-based joint resource and path selection algorithm. We show that path switching and network-aware resource selection can reduce burst loss probability and average completion time of grid jobs compared to the algorithms that are separately selecting paths and grid resources. In addition to joint resource and path selection, we present an adaptive offset algorithm for grid bursts which minimizes the average completion time. We show that the adaptive offset based QoS mechanism significantly reduces the job completion times by exploiting the trade-off between decreasing loss probability and increasing delay as a result of the extra offset time.
Open Access
Routing, spectrum allocation and regenerator placement in flexible-grid optical networks
(2013) Kahya, Alper
Tremendous increase in the number of wireless devices has been resulting in huge growth in the Internet traffic. This growth necessitates efficient usage of resources in the optical networks, which form the backbone of the Internet. Recently proposed flexible optical networks can adjust the optical layer transmission parameters to take advantage of existing channel conditions thereby increasing the resource utilization efficiency. Therefore, flexible optical network is a promising solution to fulfill growing future demand of IP traffic. Apart from efficient usage of the optical spectrum, the degradation of the optical signal as it propagates over the fiber is another problem. In such cases, the optical signal must be regenerated when a lightpath travels longer than the maximum optical reach. However, regenerators are expensive devices with high operational costs. Therefore, they should be placed carefully to reduce the capital and operational network costs. In this dissertation, we deal with the joint routing, spectrum allocation and regenerator placement (RSA-RP) problem for flexible optical networks. Our aim is to find the route and allocate spectrum for each traffic demand by assigning minimum number of nodes as regenerator sites. Firstly, we introduce a novel mixed integer linear programming (MILP) formulation for the joint RSA-RP problem. Since this formulation is not practical for large networks, we propose a decoupled formulation where the RSA-RP problem is decomposed into two phases. In the first step, we find routes and locations of regenerators assuming a full wavelength converting network. Then, we allocate the spectrum to each demand in the second phase. The decoupled model can be used to solve the RSA-RP problem for reasonably sized optical networks. We show that the decoupled model can find optimum solutions for 92% of the all cases tested for the NSFNET topology and 99% of the all cases tested for the Deutsche Telecom topology. We also show that the locations of regenerator sites significantly depend on network parameters such as the node degree and lengths of the links adjacent to the node.