The extent to which resource allocation policies are entrusted to user-level software determines in large part the degree of flexibility present in an operating system. In Hydra the determination to separate mechanism and policy is established as a basic design principle and is implemented by the construction of a kernel composed (almost) entirely of mechanisms. This paper presents three such mechanisms (scheduling, paging, protection) and examines how external policies which manipulate them may be constructed. It is shown that the policy decisions which remain embedded in the kernel exist for the sole purpose of arbitrating conflicting requests for physical resources, and then only to the extent of guaranteeing fairness.
资源分配策略委托给用户级软件的程度在很大程度上决定了操作系统中存在的灵活性程度。在Hydra中,将机制与策略分开的确定被确立为基本设计原则,并通过构造(几乎)完全由机制组成的内核来实现。本文介绍了三种这样的机制(调度,分页,保护),并研究了如何构造操纵它们的外部策略。结果表明,保留在内核中的策略决策的存在仅是为了仲裁对物理资源的冲突请求,然后仅在保证公平的程度上进行仲裁。 P>
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