Xinuo Chen

Hybrid performance-oriented scheduling of moldable jobs with QoS demands in multiclusters and grids

This paper addresses the dynamic scheduling of moldable jobs with QoS demands (soft-deadlines) in multiclusters. A moldable job can be run on a variable number of resources. Three metrics (over-deadline, makespan and idle-time) are combined with weights to evaluate the scheduling performance. Two levels of performance optimisation are applied in the multicluster. At the multicluster level, a scheduler (which we call MUSCLE) allocates parallel jobs with high packing potential to the same cluster; MUSCLE also takes the jobs’ QoS requirements into account and employs a heuristic to achieve performance balancing across the multicluster. At the single cluster level, an existing workload manager, called TITAN, utilizes a genetic algorithm to further improve the scheduling performance of the jobs allocated by MUSCLE. Extensive experimental studies are conducted to verify the effectiveness of the scheduling mechanism in MUSCLE. The results show that the comprehensive scheduling performance of parallel jobs is significantly improved across the multicluster.

Performance Analysis and Improvement of Overlay Construction for Peer-to-Peer Live Streaming

For single-source, single-tree-based peer-to-peer live media streaming, it is generally believed that a short (and wide) tree has a good comprehensive performance in terms of reliability and service delay. While the short tree directly benefits delay optimization, it is unclear whether such a structure maximizes tree reliability, which is sometimes more critical for a streaming Internet service. This article studies several prevalent overlay construction algorithms from the aspects of (1) service reliability, (2) service delay, and (3) protocol overhead. Two types of peer layout, bandwidth-ordered layout and time-ordered layout, are identified, and their performance is evaluated. The analytical results show that, by appropriately placing peers according to their time properties, the tree achieves a much higher degree of reliability than the depth-optimized tree. This finding motivates the design of a heap algorithm, which aims for combining the strengths of both bandwidth ordering and time ordering. It dynamically moves peers between difference layers of the tree according to a simple metric and gradually adjusts the tree toward a layout partially ordered in time and partially ordered in bandwidth. In so doing, the tree has advantages in both service reliability and delay. Extensive simulations show that this new algorithm achieves better comprehensive performance than existing algorithms.

Performance analysis and improvement of overlay construction for peer-to-peer live media streaming

For single-source, single-tree based peer-to-peer live media streaming, it is generally believed that a short (and wide) tree has a good comprehensive performance in terms of tree reliability and service delay. While the short tree directly benefits delay optimization, it is unclear whether such a structure maximizes tree reliability, which is sometimes more critical for a streaming Internet service. This paper studies several prevalent overlay construction algorithms in terms of (I) service reliability; (2) service delay and (3) protocol overhead. Two types of peer layout, bandwidth-ordered layout and time-ordered layout, are identified and their performance is evaluated. The analytical results show that, by appropriately placing peers according to their time properties, the tree can be much more reliable than a depth-optimized tree. We therefore propose a heap algorithm, which aims for combining the strengths of both bandwidth ordering and time ordering. It dynamically moves peers between difference layers of the tree according to a simple metric, and gradually adjusts the tree toward a layout partially ordered in time and partially ordered in bandwidth. In so doing the tree has advantages in both service reliability and delay, and maintains small protocol overheads. Extensive simulations demonstrate the effectiveness of this new algorithm.

Analysing BitTorrent's Seeding Strategies

BitTorrent is a typical peer-to-peer (P2P) file distribution application that has gained tremendous popularity in recent years. A considerable amount of research exists regarding BitTorrent’s choking algorithm, which has proved to be effective in preventing freeriders. However, the effect of the seeding strategy on the resistance to freeriders in BitTorrent has been largely overlooked. In addition to this, a category of selfish leechers (termed exploiters), who leave the overlay im-mediately after completion, has never been taken into account in previous research. In this paper two popular seeding strate-gies, the Original Seeding Strategy (OSS) and the Time-based Seeding Strategy (TSS), are chosen and we study, via mathe-matical models and simulation, their effects on freeriders and exploiters in BitTorrent networks. The mathematical model is verified and we discover that both freeriders and exploiters impact on system performance, despite the seeding strategy that is employed. However, a selfish-leecher’s threshold is identified; once this threshold is exceeded, we find that TSS outperforms OSS – that is, TSS reduces the negative impact of selfish lechers more effectively than OSS. Based on these results we discuss the choice of seeding strategy and speculate as to how more effective BitTorrent-based file distribution appli-cations can be built.

Queueing network-based optimisation techniques for workload allocation in clusters of computers

This paper addresses workload allocation techniques for clusters of computers. The workload in question is homogenous or heterogeneous. Homogeneous workload contains only QoS-demanding jobs (QDJ) or nonQoS jobs (NQJ) while heterogeneous workload is a mix of QDJs and NQJs. The processing platform used is a single cluster or multiple clusters of computers. Two workload allocation strategies (called ORT and OMR) are developed for homogeneous workloads by establishing and numerically solving optimisation equation sets. The ORT strategy achieves the optimised mean response time for homogeneous NQJ workload; while the OMR strategy achieves the optimised mean miss rate for homogeneous QDJ workload. Based on ORT and OMR, a heterogeneous workload allocation strategy is developed to dynamically partition the clusters into two parts. Each part is managed by ORT or OMR to exclusively process NQJs or QDJs. The judicial partitioning achieves an optimised comprehensive performance, which combines the mean response time and the mean miss rate. The effectiveness of these workload allocation techniques is demonstrated through queueing-theoretical analysis as well as through experimental studies. These techniques can be applied to e-business workload management to improve the distribution of different types of requests in clusters of servers.

Distributed Arbitrary Segment Trees: Providing Efficient Range Query Support over Public DHT Services

In this paper we define a Distributed Arbitrary Segment Tree (DAST), a distributed tree-like structure that layers the range query processing mechanism over public Distributed Hash Table (DHT) services. Compared with traditional segment trees, the arbitrary segment tree used by a DAST reduces the number of key-space segments that need to be maintained, which in turn results in fewer query operations and lower overheads. Moreover, considering that range queries often contain redundant entries that the clients do not need, we introduce the concept of accuracy of results (AoR) for range queries. We demonstrate that by adjusting AoR, the DHT operational overhead can be improved. DAST is implemented on a well-known public DHT service (OpenDHT) and validation through experimentation and supporting simulation is performed. The results demonstrate the effectiveness of DAST over exiting methods.

ppBLAST: A Computational Service over Peer-to-Peer Network for BLAST

BLAST is a popular toolset for protein and nucleotide sequence comparisons. Recent estimates show that these databases of protein and nucleotide sequences are set to grow exponentially and as a result local BLAST searches on single PCs cannot satisfy recent data processing requirements. Traditional approaches for the parallelisation of BLAST are focused on utilising clusters. However, scaling clusters to adapt potential growing BLAST requests will significantly increase the cost and complexities of maintenance and administration. This paper introduces ppBLAST, a BLAST service utilising the free computing resources in the peer-to-peer overlay on the Inter-net. ppBLAST uses several peer-to-peer technologies such as DAST, DHT and BitTorrent-like distribution and layered de-sign. In this paper, the feasibility of ppBLAST is validated through a small-scale deployment. The performance of ppBLAST is evaluated; we show that the execution time of a BLAST job can be considerably shortened if the scale of the supporting peer-to-peer overlay is large enough.

ModellingWeb transfer Performance over Asymmetric Networks

TCP over bandwidth asymmetric networks such as cable TV, asymmetric digital subscriber loop (ADSL) and wireless networks exhibit different characteristics from TCP on symmetric links. A number of techniques have been proposed to address this problem. However, previous research has been largely focused on bulk transfers. This paper investigates the effects of bandwidth asymmetry on Web-like short-lived transfers. A close-form prediction model is presented for TCP transfers over asymmetric links. The Web transfer model is then derived from it. Simulations based on ns-2 show that the model can give predictions for TCP transfers with a high degree of accuracy