. We incorporate time into an interleaving model of concurrency. In timed transition systems, the qualitative fairness requirements of traditional transition system are replaced (and superseded) by quantitative lower-bound and upperbound timing constraints on transitions. The purpose of this paper is to explore the scope of applicability for the abstract model of timed transition systems. We demonstrate that the model can represent a wide variety of phenomena that routinely occur in conjunction with the timed execution of concurrent processes. Our treatment covers both processes that are executed in parallel on separate processors and communicate either through shared variables or by message passing, and processes that time-share a limited number of processors under a given scheduling policy. Often it is this scheduling policy that determines if a system meets its real-time requirements. Thus we explicitly address such questions as time-outs, interrupts, static and dynamic priorities. ...

We extend the specification language of temporal logic, the corresponding verification framework, and the underlying computational model to deal with real-time properties of reactive systems. The abstract notion of timed transition systems generalizes traditional transition systems conservatively: qualitative fairness requirements are replaced (and superseded) by quantitative lower-bound and upper-bound timing constraints on transitions. This framework can model realtime systems that communicate either through shared variables or by message passing and real-time issues such as timeouts, process priorities (interrupts), and process scheduling. We exhibit two styles for the specification of real-time systems. While the first approach uses time-bounded versions of the temporal operators, the second approach allows explicit references to time through a special clock variable. Corresponding to the two styles of specification, we present and compare two different proof methodologies for t...

This paper studies the relationship between parallel computation cost and communication cost for performing divide-and-conquer (D&C) computations on a parallel system of p processors. The parallel computation cost is the maximal number of the D&C nodes that any processor in the parallel system may expand, whereas the communication cost is the total number of cross nodes. A cross node is a node which is generated by one processor but expanded by another processor. A new scheduling algorithm is proposed, whose parallel computation cost and communication cost are at most dN=pe and pdh, respectively, for any D&C computation tree with N nodes, height h, and degree d. Also, lower bounds on the communication cost are derived. In particular, it is shown that for each scheduling algorithm and for each positive ffl C ! 1, which can be arbitrarily close to 0, there are values of N , h, d, p, and ffl T (? 0), for which if the parallel computation cost is between N=p (the minimum) and (1 + ffl T ...

Fairness is a mathematical abstraction: in a multiprogramming environment, fairness abstracts the details of admissible ("fair") schedulers; in a distributed environment, fairness abstracts the relative speeds of processors. We argue that the standard definition of fairness often is unnecessarily weak and can be replaced by the stronger, yet still abstract, notion of finitary fairness. While standard weak fairness requires that no enabled transition is postponed forever, finitary weak fairness requires that for every computation of a system there is an unknown bound k such that no enabled transition is postponed more than k consecutive times. In general, the finitary restriction n(F) of any given fairness requirement F is the union of all !-regular safety properties contained in F. The adequacy of the proposed abstraction is shown in two ways. Suppose we prove a program property under the assumption of finitary fairness. In a multiprogramming environment, the program then satisfies the property for all fair finite-state schedulers. In a distributed environment, the program then satisfies the property for all choices of lower and upper bounds on the speeds (or timings) of processors. The bene ts of nitary fairness are twofold. First, the proof rules for verifying liveness properties of concurrent programs are simplified: well-founded induction over the natural numbers is adequate to prove termination under finitary fairness. Second, the fundamental problem of consensus in a faulty asynchronous distributed environment can be solved assuming finitary fairness.

Bounded fairness, a stronger notion than the usual fairness based on eventuality, can be used, for example, to relate the frequency of shared resource access of a particular process with regard to other processes that access the resource with mutual exclusion. We formalize bounded fairness byintroducing a new binary operator into temporal logic. One main di#erence between this logic and explicit-time logics, one whichwe consider to be an advantage in many cases, is that time does not appear explicitly as a parameter. The syntax and semantics for this new logic,kTL,aregiven. This logic is shown to be more powerful than temporal logic with the eventualityoperator and as powerful as the logic with the until operator. We argue that kTL can be used to specify bounded fairness requirements in a more natural manner than is possible with until; in particular, we show properties that can be expressed more succinctly in kTL. We also give a procedure for testing satis#abilityofkTL formulas. A...

ti j/t d' ()h,icclive: The objective of the Chief project is to provide an i.tegrated simulation environment for studying and evaluating various issues in designing parallel systems, including machine architectures, parallc]izillg compiler teclmiques, and parallel algorithms. The objective of the Della project is to provide a facility to allow rapid prototyping of parallelizing compilers that ca. target toward diffcrent machitm architectures.