David J. Yates

Measuring the Behavior of a World-Wide Web Server

Abstract Server performance has become a crucial issue for improving the overall performance of the World-Wide Web. This paper describes Webmonitor, a tool for evaluating and understanding server performance, and presents new results for a realistic workload. Webmonitor measures activity and resource consumption, both within the kernel and in HTTP processes running in user space. Webmonitor is implemented using an efficient combination of sampling and event-driven techniques that exhibit low overhead. Our initial implementation is for the Apache World-Wide Web server running on the Linux operating system. We demonstrate the utility of Webmonitor by measuring and understanding the performance of a Pentium-based PC acting as a dedicated WWW server. Our workload uses a file size distribution with a heavy tail. This captures the fact that Web servers must concurrently handle some requests for large audio and video files, and a large number of requests for small documents, containing text or images. Our results show that in a Web server saturated by client requests, over 90% of the time spent handling HTTP requests is spent in the kernel. Furthermore, keeping TCP connections open, as required by TCP, causes a factor of 2-9 increase in the elapsed time required to service an HTTP request. Data gathered from Webmonitor provide insight into the causes of this performance penalty. Specifically, we observe a significant increase in resource consumption along three dimensions: the number of HTTP processes running at the same time, CPU utilization, and memory utilization. These results emphasize the important role of operating system and network protocol implementation in determining Web server performance.

Parallelized network security protocols

Security and privacy are growing concerns in the Internet community, due to the Internets rapid growth and the desire to conduct business over it safely. This desire has led to the advent of several proposals for security standards, such as secure IP, secure HTTP, and the Secure Socket Layer. All of these standards propose using cryptographic protocols such as DES and RSA. Thus, the need to use encryption protocols is increasing. Shared-memory multiprocessors make attractive server platforms, for example as secure World-Wide Web servers. These machines are becoming more common, as shown by recent vendor introductions of platforms such as SGIs Challenge, Suns SPARCCenter, and DECs AlphaServer. The spread of these machines is due both to their relative ease of programming and their good price/performance. This paper is an experimental performance study that examines how encryption protocol performance can be improved by using parallelism. We show linear speedup for several different Internet-based cryptographic protocol stack running on a symmetric shared-memory multiprocessor using two different approaches to parallelism.

Cache behavior of network protocols

In this paper we present a performance study of memory reference behavior in network protocol processing, using an Internet-based protocol stack implemented in the x-kernel running in user space on a MIPS R4400-based Silicon Graphics machine. We use the protocols to drive a validated execution-driven architectural simulator of our machine. We characterize the behavior of network protocol processing, deriving statistics such as cache miss rates and percentage of time spent waiting for memory. We also determine how sensitive protocol processing is to the architectural environment, varying factors such as cache size and associativity, and predict performance on future machines.We show that network protocol cache behavior varies widely, with miss rates ranging from 0 to 28 percent, depending on the scenario. We find instruction cache behavior has the greatest effect on protocol latency under most cases, and that cold cache behavior is very different from warm cache behavior. We demonstrate the upper bounds on performance that can be expected by improving memory behavior, and the impact of features such as associativity and larger cache sizes. In particular, we find that TCP is more sensitive to cache behavior than UDP, gaining larger benefits from improved associativity and bigger caches. We predict that network protocols will scale well with CPU speeds in the future.

Predictors of on-line services and e-participation: A cross-national comparison

Abstract Effective e-government creates an environment for citizens to have greater access to their government and, in theory, makes citizen-to-government contact more inclusive. Our research examines two distinct but related measures of e-government effectiveness, namely the online service index and the e-participation index, both reported in the 2010 e-government survey conducted by the United Nations. We analyze the impact of political structure, public sector performance and policy initiatives on both indices in more than 150 countries. Our multiple regression analysis shows that there is greater e-government capability in countries that have more effective public sector governance and administration, and policies that advance the development and diffusion of information and communication technologies. More democratic institutions and processes, however, appear to have a negative impact on e-government. In addition, countries that practice effective governance and promote competition in the telecommunications sector demonstrate more extensive provision of e-participation. These results suggest that the path to e-government leverages different strategies depending on a nations political structure, and that authoritarian countries may be utilizing e-government to maintain the status quo.

Networking support for large scale multiprocessor servers

Over the next several years the performance demands on globally available information servers are expected to increase dramatically. These servers must be capable of sending and receiving data over hundreds or even thousands of simultaneous connections. In this paper, we show that connection-level parallel protocols (where different connections are processed in parallel) running on a shared-memory multiprocessor can deliver high network bandwidth across a large number of connections.We experimentally evaluate connection-level parallel implementations of both TCP/IP and UDP/IP protocol stacks. We focus on three questions in our performance evaluation: how throughput scales with the number of processors, how throughput changes as the number of connections increases, and how fairly the aggregate bandwidth is distributed across connections. We show how several factors impact performance: the number of processors used, the number of threads in the system, the number of connections assigned to each thread, and the type of protocols in the stack (i.e., TCP versus UDP).Our results show that with careful implementation connection-level parallel protocol stacks scale well with the number of processors, and deliver high throughput which is, for the most part, sustained as the number of connections increases. Maximizing the number of threads in the system yields the best overall throughput. However, the best fairness behavior is achieved by matching the number of threads to the number of processors and scheduling connections assigned to threads in a round-robin manner.

On Per-Session End-to-End Delay and the Call Admission Problem for Real-Time Applications with QOS Requirements

A crucial problem facing the designers and deployers of future high-speed networks is providing applications with quality of service (QOS) guarantees. For soft real-time applications, which are delay sensitive but loss tolerant, delay distribution is an important QOS measure of interest. In this paper we study (through simulation) the end-to-end delay distribution seen by individual sessions under simple first-come-first-served (FCFS) multiplexing in a network model with two significant features: (1) all traffic is connection-oriented, (2) cross traffic along routes is representative of that seen by calls in a moderately sized wide area network (i.e., less than 100 switches). We compare these delay distributions with the worst case analytic delay bounds predicted by three different techniques for providing such bounds (two of which require a more sophisticated link-level scheduling policy). We also consider the per-hop delay distributions Seen as a session progresses “deeper” into the network and determine the sensitivity of these delay distributions to the manner in which the interfering traffic is modeled. Finally, we use our delay distribution results to examine the tradeoff between the QOS requested by a call, the manner in which the QOS guarantee is provided, and the number of calls that are admitted at the requested QOS.

Different Paths to Broadband Access: The Impact of Governance and Policy on Broadband Diffusion in the Developed and Developing Worlds

A new digital divide is emerging both within and between nations that is due to inequalities in broadband Internet access. Our research examines the broadband digital divide by analyzing the impact of administrative culture and policy initiatives in the form of strategic planning, execution, regulation and investment on broadband diffusion in 139 countries. Our multiple regression analysis shows that factors that determine broadband diffusion in technologically developed countries do not necessarily have the same impact in less developed countries. For example, competition in the telecommunications sector has a positive impact in nations where access to information and communication technologies (ICTs) is expanding, but does not make a significant difference where ICT access is widely available. We also show that when controlling for measures of economic, political, social and educational development, there is greater broadband diffusion in countries that have an administrative culture of sound governance and make a higher shared financial investment in information and communication technologies. These results hold in nations where access to ICTs is expanding, even though the presence of a national telecommunications regulatory authority has a negative impact on broadband diffusion in the same group of countries. Our results suggest that the path to widespread availability and use of broadband requires different strategies depending on a nations level of technological development. Furthermore, assessing overall government performance in terms of governance and policy initiatives on this journey is more important than factors such as the presence or absence of a national regulatory authority.

Data Quality and Query Cost in Pervasive Sensing Systems

This research is motivated by large-scale pervasive sensing applications. We examine the benefits and costs of caching data for such applications. We propose and evaluate several approaches to querying for, and then caching data in a sensor field data server. We show that for some application requirements (i.e., when delay drives data quality), policies that emulate cache hits by computing and returning approximate values for sensor data yield a simultaneous quality improvement and cost savings. This win-win is because when system delay is sufficiently important, the benefit to both query cost and data quality achieved by using approximate values outweighs the negative impact on quality due to the approximation. In contrast, when data accuracy drives quality, a linear trade-off between query cost and data quality emerges. We also identify caching and lookup policies for which the sensor field query rate is bounded when servicing an arbitrary workload of user queries. This upper bound is achieved by having multiple user queries share the cost of a single sensor field query. Finally, we demonstrate that our results are robust to the manner in which the environment being monitored changes using models for two different sensing systems.

Understanding the Impact of Political Structure, Governance and Public Policy on E-Government

Effective e-government creates an environment for citizens to have greater access to their government and, in theory, makes citizen-to-government contact more inclusive. Our research examines two distinct but related measures of e-government effectiveness, namely the online service index and the e-participation index, both reported in the 2010 e-government survey conducted by United Nations. We analyze the impact of political structure, administrative culture and policy initiatives on both indices in approximately 160 countries. Our multiple regression analysis shows that when controlling for measures of economic and educational development, there is greater e-government capability in countries that have an administrative culture of sound governance and policies that promote the development and diffusion of information and communication technologies. These results hold in nations that are more democratic, even though political freedom (i.e., press freedom and civil liberties) appears to have a negative impact on e-government. These results suggest that the path to e-government leverages different strategies depending on a nations political structure, and that countries in which there is less political freedom may be utilizing e-government to maintain the status quo.

Towards E-participation in the Middle East and Northern Europe

E-governance is important for enabling governments to communicate with and serve their citizens. In this chapter, we determine critical success factors for and obstacles to effective e-governance in two very different regions of the world, namely, the Middle East and Northern Europe. Specifically, we explore e-participation in Bahrain, Egypt, Estonia, and Finland. These four countries are interesting cases because they have quite different types of government, yet, according to recent United Nations data, each has succeeded in their development of e-government capabilities. However, the degree to which this development has translated into effective e-participation varies significantly between these countries. We propose a framework for assessing the strategy, policies, and context within which e-governance is being developed and apply this framework to examine e-participation in each country. Using this framework, we show that an administrative culture of high-quality governance is critical to the success of e-participation initiatives. Our research also suggests that factors which promote online citizen participation in the political process include a government commitment to e-government, policies that encourage e-participation, and investment in e-governance and information and communication technology (ICT) development. Sometimes these conditions are satisfied but e-participation falls short of expectations. We present evidence that this short-fall often reflects problems in the offline relationship between governments and their citizens.