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By Raphael A. Finkel

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Даны основы организации, идеологии и архитектуры, объединяющие различные версии UNIX. Рассматриваются: архитектура ядра (подсистемы ввода/вывода, управления памятью и процессами, а также файловая подсистема), программный интерфейс (системные вызовы и основные библиотечные функции), пользовательская среда (командный интерпретатор shell, основные команды и утилиты) и сетевая поддержка (протоколы семейства ТСР/IР, архитектура сетевой подсистемы, программные интерфейсы сокетов и TLI).

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Non-preemptive policies are an example of the Hysteresis Principle, which we will encounter repeatedly. ❅❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂✂❂ ❅ Hysteresis Principle ❅ ❅ ❃✂ ❅ ❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃✂❃ ❅ ❅ Resist change. ❅ ❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄✂❄ All change has a cost. In our case, preemption involves switching to a new process, which requires updating software tables and often hardware tables as well.

The quantum given to any process is based on the queue it is taken from. A process that needs a long time suffers process switches after times q , 3q , 7q , 15q , and so on. The total number of process switches is therefore log(t (p ) / q ) instead of t (p ) / q , which is the number needed by RR. Therefore, this method reduces process switch overhead while still behaving much like RR. q . Such functions keep the quantum size within reasonable bounds while still reducing the total number of process switches needed for long processes.

Three common approaches are analysis, simulation, and experimentation. Analysis involves a mathematical formulation of the policy and a derivation of its behavior. 14. This picture shows a simplified model of the transitions through which a process might pass. Lines indicate transitions, rectangular objects represent queues, and circles represent servers. The model in this figure shows a multiprocessor with two cpu’s that share a single queue. A process (called a ‘‘customer’’ in the jargon of queueing theory) enters from the right and is queued for execution.

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