Over the past decade, researches on operating systems have been shifted from monolithic kernels towards microkernels for several reasons. Of them are: (1) It is easier to understand and debug because the kernel is much smaller. (2) It is much more secure and flexible because the kernel is small, and most of the kernel functions can be implemented as servers running in the user space instead of in the kernel space. (3) The message passing technique that is in the core of the microkernel has been improved, and thus the overhead required for the message passing has been highly reduced. Minix 3 is one of the microkernels developed over the past decade and is aimed for educational purpose and small PCs. It is implemented on the IA-32 architecture and is based on the segmented memory model of IA-32. The purpose of this thesis is to use Minix 3 as a case study and to convert the segmented memory model adopted by the current implementation to coexisted with the demand paged memory model, which is also supported by the IA-32 architecture. That said, the thesis can be divided into two parts: The first part is to implement a new server called pager, which would take over the memory management subsystem of the Minix 3 microkernel and be used to offload the overhead of the kernel. The second part is to implement a virtual memory management subsystem that uses the segmentation with paging memory model of the IA-32 architecture.

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Time-shared, virtual memory systems are very complex and changes in their performance may be caused by many factors - by variations in the workload as well as changes in system configuration. The evaluation of these systems can thus best be carried out by linking results obtained from a planned programme of measurements, taken on the system, to some model of it. Such a programme of measurements is best carried out under conditions in which all the parameters likely to affect the system's performance are reproducible, and under the control of the experimenter. In order that this be possible the workload used must be simulated and presented to the target system through some form of automatic workload driver. A case study of such a methodology is presented in which the system (in this case the Edinburgh Multi-Access System) is monitored during a controlled experiment (designed and analysed using standard techniques in common use in many other branches of experimental science) and the results so obtained used to calibrate and validate a simple simulation model of the system. This model is then used in further investigation of the effect of certain system parameters upon the system performance. The factors covered by this exercise include the effect of varying: main memory size, process loading algorithm and secondary memory characteristics.

Current computer systems fill the demand of operating systems and applications for ever greater amounts of random access memory by paging the least recently used data to the hard disk drive. This paging process is called "virtual memory," to indicate that the hard disk drive is used to create the illusion that the computer has more random access memory than it actually has. Unfortunately, the fastest hard disk drives are over five orders of magnitude slower than the DRAM they are emulating. When the demand for memory increases to the point that processes are being continually saved to disk and then retrieved again, a process called "thrashing" occurs, and the performance of the entire computer system plummets. This thesis sought to find alternatives for home and small business computer users to the hard disk drive for virtual memory which would not suffer from the same long delays. Virtual memory is especially important for older computers, which often are limited by their motherboards, their processors and their power supplies to a relatively small amount of random access memory. Thus, this thesis was focused on improving the performance of older computers by replacing the hard disk drive with faster technologies for the virtual memory. Of the different technologies considered, flash memory was selected because of its low power requirements, its ready availability, its relatively low cost and its significantly faster random access times. Two devices were evaluated on a system with a 512MB of RAM, a Pentium 4 processor and a SATA hard disk drive. Theoretical models and a simulator were developed, and physical performance measurements were taken. Flash memory was not shown to be significantly faster than the hard disk drive in virtual memory applications.

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No description available.

Systems that support virtual memory virtualize the available physical memory such that the applications running on them operate under the assumption that these systems have a larger amount of memory available than is actually present. The memory managers of these systems manage the virtual and the physical address spaces and are responsible for converting the virtual addresses used by the applications to the physical addresses used by the hardware. The memory managers assume that the amount of physical memory is constant and does not change during their period of operation. Some operating scenarios however, such as the power conservation mechanisms and virtual machine monitors, require the ability to vary the physical memory available at runtime, thereby making invalid the assumptions made by these memory managers. In this work we evaluate the suitability of the Linux Memory Manager, which assumes that the available physical memory is constant, for the purposes of varying the memory at run time. We have implemented an infrastructure over the Linux 2.6.11 kernel that enables the user to vary the physical memory available to the system. The available physical memory is logically divided into banks and each bank can be turned on or off independent of the others, using the new system calls we have added to the kernel. Apart from adding support for the new system calls, other changes had to be made to the Linux memory manager to support the runtime variation of memory. To evaluate the suitability for varying memory we have performed experiments with varying memory sizes on both the modified and the unmodified kernels. We have observed that the design of the existing memory manager is not well suited to support the runtime variation of memory; we provide suggestions to make it better suited for such purposes. Even though applications running on systems that support virtual memory do not use the physical memory directly and are not aware of the physical addresses they use, the amount of physical memory available for use affects the performance of the applications. The results of our experiments have helped us study the influence the amount of physical memory available for use has on the performance of various types of applications. These results can be used in scenarios requiring the ability to vary the memory at runtime to do so with least degradation in the application performance. / Master of Science

O desempenho do sistema de memória virtual depende diretamente da qualidade da política de gerência de memória. Estratégias podem ser desenvolvidas para melhorar tal desempenho: uma delas é criar novas políticas de gerência de memória que tenham, ao mesmo tempo, bom desempenho e simplicidade; outra maneira é desenvolver técnicas e incluir informações para auxiliar as políticas já existentes. Este trabalho procura mostrar uma estratégia para auxiliar políticas de substituição com a finalidade de obter bom desempenho em um sistema de gerência de memória, sem a necessidade de alterar o comportamento da política de substituição. Para isso, foi utilizada a técnica de busca antecipada de páginas em conjunto com a informação de frequência de acessos, obtida por meio de um método usado em processamento estatístico de linguagem natural. Os resultados mostram, além do bom desempenho, que a mesma estratégia pode ser adotada em qualquer algoritmo. / The virtual memory system performance depends directly on the quality of the memory management policy. Strategies can be developed to improve such performance: one of them is creating new memory management policies that present, at the same time, simplicity and good performance; another one is developing techniques and include information that will aid the policies that already exist. This paper aims to show a strategy that will aid replacement policies in order to obtain a good performance in a memory management system without changing the replacement policy behavior. To do so, a page prefetching technique along with information about access frequency, obtained through a method used in a statistical natural language processing, was used. The results show, besides the good performance, that the same strategy can be adopted in any algorithm.

Apresenta uma reflexão a respeito das representações sociais e sua importância na construção das memórias virtuais da cidade a partir das informações publicadas por usuários em um aplicativo de celular. Estudo realizado no âmbito do projeto de pesquisa Porto Alegre Imaginada e articula conceitos como informação, representações sociais e memória virtual, a partir de uma perspectiva teórica interdisciplinar. Defende o estudo da cidade a partir das representações que os cidadãos possuem sobre ela, amparadas em suas experiências com o ambiente urbano. Enfatiza o estudo das representações sociais a partir das informações produzidas pelos cidadãos, pois elas são amparadas na memória e marcadas pela subjetividade dos indivíduos. Objetiva compreender como informações produzidas pelos cidadãos no aplicativo Foursquare auxiliam na constituição das representações sobre Porto Alegre e na dinamização das suas memórias. Afirma que, ao produzirem e compartilharem informações sobre a cidade, os cidadãos atuam como narradores do ambiente urbano Analisa as dicas publicadas pelos cidadãos no aplicativo nas páginas de Porto Alegre e do Bom Fim, Centro Histórico, Cidade Baixa, Menino Deus e Moinhos de Vento, que são bairros citados pelos usuários na página da cidade. Pesquisa qualitativa que utiliza a narratologia como método para analisar os temas, os cenários, as personagens, os enredos e as sequências cronológicas que compõem as histórias narradas pelos cidadãos no aplicativo. Constrói mapas para localizar os lugares dos cenários da cidade citados pelos cidadãos nas narrativas. As narrativas formadas pelas dicas dos cidadãos representam uma cidade múltipla e envolta por tensões, embora a memória virtual seja selecionada e enquadrada de acordo com as concepções de cada sujeito. Conclui que o aplicativo dinamiza a memória social e a construção de um imaginário sobre a apropriação do espaço vivido. / It presents a reflection about the social representations and their importance in the construction of the virtual memories of the city from the information published by users in a mobile app. This study was carried out within the scope of the Porto Alegre Imaginada research project and which articulates concepts such as information, social representations and virtual memory, from an interdisciplinary theoretical perspective. It defends the study of the city from the representations that the citizens have on her, supported in their experiences with the urban environment. Emphasizes the study of social representations based on the information produced by citizens, because they are supported in memory and marked by the subjectivity of individuals. It aims to understand how information produced by citizens in the Foursquare app helps in the constitution of representations about Porto Alegre and the dynamization of their memories Afirms that in producing and sharing information about the city, citizens act as narrators of the urban environment. Analyzes the tips published by citizens in the app in the pages of Porto Alegre and Bom Fim, Centro Histórico, Cidade Baixa, Menino Deus and Moinhos de Vento, which are neighborhoods mentioned by users on the city page. Qualitative research that uses narratology as a method to analyze the themes, scenarios, characters, scenarios and chronological sequences that make up the stories narrated by the citizens in the app. Constructs maps to locate the places of the city's scenarios cited by the citizens in the narratives. The narratives formed by the tips of the citizens represent a multiple city and surrounded by tensions, although the virtual memory is selected and framed according to the conceptions of each subject. Concludes that the app dynamizes the social memory and the construction of an imaginary about an appropriation of the lived space.

O desempenho do sistema de memória virtual depende diretamente da qualidade da política de gerência de memória. Estratégias podem ser desenvolvidas para melhorar tal desempenho: uma delas é criar novas políticas de gerência de memória que tenham, ao mesmo tempo, bom desempenho e simplicidade; outra maneira é desenvolver técnicas e incluir informações para auxiliar as políticas já existentes. Este trabalho procura mostrar uma estratégia para auxiliar políticas de substituição com a finalidade de obter bom desempenho em um sistema de gerência de memória, sem a necessidade de alterar o comportamento da política de substituição. Para isso, foi utilizada a técnica de busca antecipada de páginas em conjunto com a informação de frequência de acessos, obtida por meio de um método usado em processamento estatístico de linguagem natural. Os resultados mostram, além do bom desempenho, que a mesma estratégia pode ser adotada em qualquer algoritmo. / The virtual memory system performance depends directly on the quality of the memory management policy. Strategies can be developed to improve such performance: one of them is creating new memory management policies that present, at the same time, simplicity and good performance; another one is developing techniques and include information that will aid the policies that already exist. This paper aims to show a strategy that will aid replacement policies in order to obtain a good performance in a memory management system without changing the replacement policy behavior. To do so, a page prefetching technique along with information about access frequency, obtained through a method used in a statistical natural language processing, was used. The results show, besides the good performance, that the same strategy can be adopted in any algorithm.