Scheduling for today’s computer systems: bridging theory and practice

Scheduling is a fundamental technique for improving performance in computer systems. From web servers to routers to operating systems, how the bottleneck device is scheduled has an enormous impact on the performance of the system as a whole. Given the immense literature studying scheduling, it is easy to think that we already understand enough about scheduling. But, modern computer system designs have highlighted a number of disconnects between traditional analytic results and the needs of system designers. In particular, the idealized policies, metrics, and models used by analytic researchers do not match the policies, metrics, and scenarios that appear in real systems. The goal of this thesis is to take a step towards modernizing the theory of scheduling in order to provide results that apply to today’s computer systems, and thus ease the burden on system designers. To accomplish this goal, we provide new results that help to bridge each of the disconnects mentioned above. We will move beyond the study of idealized policies by introducing a new analytic framework where the focus is on scheduling heuristics and techniques rather than individual policies. By moving beyond the study of individual policies, our results apply to the complex hybrid policies that are often used in practice. For example, our results enable designers to understand how the policies that favor small job sizes are affected by the fact that real systems only have estimates of job sizes. In addition, we move beyond the study of mean response time and provide results characterizing the distribution of response time and the fairness of scheduling policies. These results allow us to understand how scheduling affects QoS guarantees and whether favoring small job sizes results in large job sizes being treated unfairly. Finally, we move beyond the simplified models traditionally used in scheduling research and provide results characterizing the effectiveness of scheduling in multiserver systems and when users are interactive. These results allow us to answer questions about the how to design multiserver systems and how to choose a workload generator when evaluating new scheduling designs

Heavy tailed M/G/1-PS queues with impatience and admission control in packet networks

In this paper we analyze the M/G/1 processor sharing queue with heavy tailed services and with impatient customers. It is assumed that impatience depends on the value of the service required. We prove that a reduced service rate (RSR) approximation holds for estimating the sojourn time of a customer in the system, when the queue capacity is finite or infinite. This allows us to evaluate the reneging probability of customers with very large service times. We then use these results to investigate the impact of admission control on a link of a packet network. Admission control simply consists of limiting the number of simultaneous connections. It turns out that there is a real benefit for the efficiency of the system to perform admission control: It globally increases the fraction of customers, who complete their service (i.e. without being impatient). Finally, we investigate the fairness of the system and propose a criterion to assess the capacity of the system so as to allow the completion of very large service times

Health-seeking behaviour for childhood illnesses in urban South Africa

In urban South Africa there has been increasing child morbidity and mortality as a result of HIV/AIDS; a paucity of data on health-seeking behaviour for childhood illnesses; high reported use of traditional medicine and general patient dissatisfaction with free public health services. This study therefore aims to investigate the main factors influencing choice of health care provider for Black children under 6 years of age in Johannesburg and Soweto, using both qualitative and quantitative methods. In-depth interviews with caregivers (n = 5), providers of traditional (n = 6) and Western (n=6) health care, as well as 5 focus groups with caregivers, provide insight into different health care beliefs and practices. An utilisation-based survey was conducted with 206 Black caregivers of children under 6 years of age from 1 public clinic in Soweto (n = 50), 2 private clinics (50 caregivers in total) in Johannesburg, 2 public hospitals (53 caregivers in total) from Johannesburg and Soweto and 2 traditional healers (53 caregivers in total) from Johannesburg and Orange Farm, an informal settlement on the outskirts of Johannesburg. The facilities where interviews took place in this study were purposively selected. Caregiver beliefs were found to be at the heart of the decision-making process. These beliefs are shaped by the caregiver's world-view, religion, family (particularly grandmothers of the child and caregiver), social networks and previous experiences. Beliefs were also found to affect the caregiver's perceptions of the child and the illness. Findings suggest that the caregiver will be limited in her decision-making by her age and accumulated knowledge, her socio-economic status as well as the availability of support and social networks. These in turn affect the degree to which distance and cost are barriers to health care-seeking. Characteristics of the provider, experiences in the past, as well as the outcome of the treatment, were all found to interact with the afore-mentioned factors to shape current patterns of resort, as well as future decisions made. When a child was unwell, caregivers usually gave home treatments first, particularly for diarrhoea, vomiting, fever, constipation and crying. Although the use of over-the-counter (OTC) medicines in this study varied according to the illness being treated, overall the higher socio-economic status (SES) respondents were more likely to use OTC medicines and less likely than other groups to use home treatments. Home treatments as well as OTC medicines were not always used appropriately. Nearly three-quarters of caregivers had given or would give traditional medicine to their child if the need arose, although this varied by education levels and SES. The most wellknown African childhood illnesses were inyoni and ibala, which in the South African world-view Western medicine is not able to treat. In general, private sector care (primary and higher) was perceived to be better than public sector care because of more thorough examinations, stronger medicines, enough medicines and generous prescriptions, no queues, friendly staff who listen and are attentive, the availability of a doctor, high-tech equipment, cleanliness and the food provided. Although the public hospitals suffer from similar problems to those found at PHC clinics, compared with the primary health care facilities they were generally thought to be better-stocked with medicines and more accessible in terms of opening hours, performing better physical examinations, having more doctors and high-tech equipment, and staff were believed to be more attentive than their PHC counterparts. Results from this study highlight the need for community and household integrated management of childhood illnesses {IMCI) which has remained underdeveloped in South Africa. In particular, this should involve understanding the symptoms that mothers themselves recognise as significant and incorporating these into health education messages, as well as educating caregivers (mothers, grandmothers and relatives) about the safe, timely and appropriate use of enemas, home treatments, over-the-counter medicines and traditional medicines. Furthermore, these results underline the need to improve patient and provider communication as well as patient care in public services, the need to include traditional healers in the community component of IMCI, as well as training Western health care providers about health-seeking behaviour in the context of the local belief system. In order to improve the future health and well-being of millions of children in South Africa, understanding the determinants of health-seeking behaviour is essential for ensuring that effective strategies are put in place and that changes are implemented where they are needed most.EThOS - Electronic Theses Online ServiceGBUnited Kingdo