Technical Correspondence

s from Other ACM Publications In the ACM Transactions on Computer Systems February Issue Computational Algorithms for State.Dependent Queueing Networks Charles H. Souer Queueing networks are important as performance models of computer and communication systems. Exact numerical solution of a queueing network is usually only feasible if the network has a product form solution in the sense of Jackson. Product form networks allow a rich variety of forms of state-dependent behavior. However, efficient computational algorithms have not been developed for several of the allowed forms of state-dependent behavior. This paper develops the two most important computational algorithms. Convolution and Mean Value Analysis, to apply to forms of state-dependent behavior allowed in product form networks. It is demonstrated that these two algorithms are of equal generality, contrary to common belief. For Correspondence: C. H. Sauer, IBM, Dept. 450, Bldg. 984, 11400 Burnett Rd., Austin, TX 78758. On The Generation of Cryptographically Strong Pseudorandom Sequences Adi Shamir This paper shows how to generate from a short random seed a long sequence of pseudorandom numbers which is cryptographically strong in the sense that knowing some sequence elements cannot possibly help the cryptanalyst to determine other sequence elements. The method is based on the RSA cryptosystem, and it is the first published example of a pseudorandom sequence generator for which such a property has been formally proved. For Correspondence: A. Shamir, Dept. of Applied Mathematics, The Weizmann Inst. of Science, Rehovot, Israel. Interprocess Communication and Processor Dispatching on the Inte1432 George W. Cox, william M. Corwin, Konrad K. Lai, and Fred J. Pollack A united facility for interprocess communication and processor dispatching on the lntel 432 is described. The facility is based on a queueing and binding mechanism called a port. The goals and motivations for ports, both abstract an implementation views of them, and their absolute and comparative performance are described. For Correspondence: G. W. Cox. Intel Corp., 5200 N. E. Elam Young Pkwy. , Hillsboro. OR 97123 Cache Performance in the VAX.111780 Douglas W. Clark The performance of memory caches is usually studied through tracedriven simulation. This approach has several drawbacks. Notably, it excludes realistic: multiprogramming, operating system, and 1/O activity. In this paper, cache performance is studied by direct measurement of the hardware. A hardware monitor was attached to a VAX-11/780 computer, whose cache was then measured during normal use. A reproducible synthetic timesharing workload was also run. This paper reports measurements including the hit ratios of data and instruction references, the rate of cache invalidations by I/O, and the amount of waiting time due to cache misses. Additional measurements were made with half the cache disabled, and with the entire cache disabled. For Correspondence: D. W. Clark. Digital Equipment Corp., 295 Foster St., Littleton. MA 01460. Implementing Atomic Actions on Decentralized Data David P. Reed Synchronization of accesses 1o shared data and recovering the state of such in the case of failures are really two aspects of the same problem-implementing alomic aclions on a related set of data items. In this paper a mechanism that solves both problems simultaneously in a way that is compatible with requirements of decentralized systems is described. In particular, the correct construction and execution of a new atomic action can be accomplished without knowledge of all other atomic actions in the system that might execute concurrently. Further, the mechanisms degrade gracefully if parts of the system fail: only those atomic actions that require resources in failed parts of the system are prevented from executing, and there is no single coordinator that can fail and bring down the whole system. For Correspondence: D. P. Reed, Laboratory for Computer Science, MIT, Cambridge. MA 02139. February 1983 Volume 26 Number 2 Communications of the ACM 149