With the increasing use of various Web-based services, design of high performance, scalable and dependable data centers has become a critical issue. Recent studies show that a clustered, multi-tier architecture is a cost-effective approach to design such servers. Since these servers are highly distributed and complex, understanding the workloads driving them is crucial for the success of the ongoing research to improve them. In view of this, there has been a significant amount of work to characterize the workloads of Web-based services. However, all of the previous studies focus on a high level view of these servers, and analyze request-based or session-based characteristics of the workloads. In this paper, we focus on the characteristics of the network behavior within a clustered, multi-tiered data center. Using a real implementation of a clustered three-tier data center, we analyze the arrival rate and inter-arrival time distribution of the requests to individual server nodes, the network traffic between tiers, and the average size of messages exchanged between tiers. The main results of this study are; (1) in most cases, the request inter-arrival rates follow log-normal distribution, and self-similarity exists when the data center is heavily loaded, (2) message sizes can be modeled by the log-normal distribution, and (3) service times fit reasonably well with the Pareto distribution and show heavy tailed behavior at heavy loads.

Improving the performance of Web servers has become a critical issue to handle the increasing demand on various network-based services. In this context, we exploit the local memory of programmable network interface cards (NICs) to improve the performance of cluster-based Web servers, which are increasingly used in designing Web server platforms. We use the NIC memory for caching recently accessed data blocks to improve server performance. We have implemented a prototype of the proposed NIC caching mechanism for a distributed Web server, based on PRESS (Carrera et al., 2002), on an 8-node, Myrinet-connected Linux cluster. Measurements with several server workloads show that NIC caching can enhance throughput by up to 27% compared to the original PRESS Web server without NIC caching, by minimizing the DMA and PCI bus overhead

Design of high performance Web servers has become a recent research thrust to meet the increasing demand of network-based services. In this paper, we propose a new Web server architecture, called multi-threaded PIPELINED Web server, suitable for Symmetric Multi-Processor (SMP) or System-on-Chip (SoC) architectures. The proposed PIPELINED model consists of multiple thread pools, where each thread pool consists of five basic threads and two helper threads. The main advantages of the proposed model are global information sharing by the threads, minimal synchronization overhead due to less number of threads, and non-blocking I/O operations, possible with the helper threads.We have conducted an in-depth performance analysis of the proposed server model along with four prior Web server models (Multi-Process (MP), Multi-Thread (MT), Single-Process Event-Driven (SPED) and Asynchronous Multi-Process Event-Driven (AMPED)) via simulation using six Web server workloads. The experiments are conducted to investigate the impact of various factors such as the memory size, disk speed and numbers of clients. The simulation results indicate that the proposed PIPELINED Web server architecture shows the best performance across all system and workload parameters compared to the MP, MT, SPED and AMPED models. Although the MT and AMPED models show competitive performance with less number of processors, the advantage of the PIPELINED model becomes obvious as the number of processors or clients in an SMP/SoC machine increases. The MP model shows the worst performance in most of the cases. The results indicate that the proposed server architecture can be used in future large-scale SMP/SoC machines to boost system performance.

Summary form only given. We investigate the feasibility of minimizing the response time of a Web server by exploiting the advantages of both user-level communication and coscheduling. We, thus, propose a coscheduled server model, based on the PRESS design where the remote cache accesses can be coscheduled on different nodes to reduce the response time. We experiment this concept using two known coscheduling techniques, called dynamic coscheduling (DCS) and DCS with immediate blocking. Extensive simulation of four server models (PRESS over TCP/IP, PRESS over VIA, coscheduled PRESS model with DCS, and with DCS and blocking) using 16-node and 32-node cluster configurations indicates that the average response time of a distributed server can be minimized significantly by coscheduling the communicating processes. The use of the DCS scheme reduced the average latency up to 80%, on an average 40%, compared to the PRESS over VIA model that uses only user-level communication, and by order of magnitude compared to the TCP/IP model. The throughput of the three user-level communication models is at least 25% better compared to the PRESS over TCP/IP model. Experiments with varying file size and cache size also confirmed the advantage of using a coscheduling mechanism for improving the response time behavior.

In this paper, we conduct an in-depth evaluation of a broad spectrum of scheduling alternatives for clusters. These include the widely used batch scheduling, local scheduling, gang scheduling, all prior communication-driven coscheduling algorithms (Dynamic Coscheduling (DCS), Spin Block (SB), Periodic Boost (PB), and Co-ordinated Coscheduling (CC)) and a newly proposed HYBRID coscheduling algorithm on a 16-node, Myrinet-connected Linux cluster. Performance and energy measurements using several NAS, LLNL and ANL benchmarks on the Linux cluster provide several interesting conclusions. First, although batch scheduling is currently used in most clusters, all blocking-based coscheduling techniques such as SB, CC and HYBRID and the gang scheduling can provide much better performance even in a dedicated cluster platform. Second, in contrast to some of the prior studies, we observe that blocking-based schemes like SB and HYBRID can provide better performance than spin-based techniques like PB on a Linux platform. Third, the proposed HYBRID scheduling provides the best performance-energy behavior and can be implemented on any cluster with little effort. All these results suggest that blocking-based coscheduling techniques are viable candidates to be used in clusters for significant performance-energy benefits.