一种静态最少优先级分配算法

随着实时系统越来越多地应用于各种快速更新系统,尤其是各种片上系统,如PDA(personal digital assistant),PSP(play station portable)等,性价比已成为系统设计者的主要关注点.实际应用中,实时系统通常仅支持较少的优先级,常出现系统优先级数小于任务数的情况(称为有限优先级),此时,需将多个任务分配到同一系统优先级,RM(rate monotonic),DM(deadline monotonic)等静态优先级分配算法不再适用.为此,静态有限优先级分配是研究在任务集合静态优先级可调度的情况下,可否以及如何用较少或最少的系统优先级保持任务集合可调度.已有静态有限优先级分配可分为两类:固定数目优先级分配和最少优先级分配.给出了任意截止期模型下任务静态有限优先级可调度的充要条件以及不同静态有限优先级分配间转换时的几个重要性质,指出了系统优先级从低到高分配策略的优越性,定义了饱和任务组与饱和分配的概念,证明了在任务集合静态优先级可调度的情况下,最少优先级分配比固定数目优先级分配更具一般性.最后提出一种最少优先级分配算法LNPA(least-number priority assignment).与现有算法相比,LNPA适用范围更广,且复杂度较低.

An Efficient Approach for Mitigating Covert Storage Channel Attacks in Virtual Machines by the Anti-Detection Criterion

Covert channels have been an effective means for leaking confidential information across security domains and numerous studies are available on typical covert channels attacks and defenses.Existing covert channel threat restriction solutions are based on the threat estimation criteria of covert channels such as capacity,accuracy,and short messages which are effective in evaluating the information transmission ability of a covert(storage)channel.However,these criteria cannot comprehensively reflect the key factors in the communication process such as shared resources and synchronization and therefore are unable to evaluate covertness and complexity of increasingly upgraded covert storage channels.As a solution,the anti-detection criterion was introduced to eliminate these limitations of cover channels.Though effective,most threat restriction techniques inevitably incur high performance overhead and hence become impractical.In this work,we avoid such overheads and present a restriction algorithm based on the anti-detection criterion to restrict threats that are associated with covert storage channels in virtual machines while maintaining the resource efficiency of the systems.Experimental evaluation shows that our proposed solution is able to counter covert storage channel attacks in an effective manner.Compared with Pump,a well-known traditional restriction algorithm used in practical systems,our solution significantly reduces the system overhead.