Studies of some feedback control mechanisms in operating systems

Abstract

The possibility of enhancing the effectiveness of an operating system by the introduction of appropriate feedback controls is explored by examining some resource allocation problems. The allocation of and I/O processors in a multiprogramming core, CPU demand paging environment is studied in terms of feedback control. A major part of this study is devoted to the application of feedback control concepts to core allocation to prevent thrashing and develop algorithms of practical value. To aid this study a simulator is developed which uses probability distributions to represent program behaviour. Successful algorithms are developed employing a two stage page replacement function which selects a process from which a page is then chosen to be replaced. Improving the performance of these algorithms by using a 'drain process' to aid the dynamic determination of the current locality of a process is also discussed. The complexity of the overall resource allocation problem is dealt with by employing a hierarchy of individual resource allocation policies. These control scheduling, core allocation and dispatching. By considering the levels of the hierarchy as separate feedback control systems the restrictions which must be placed upon the individual levels are derived. The extension of these results to further levels is also discussed.