Implementing Sequentially Consistent Programs on Processor Consistent Platforms

Abstract

<p>Designers of distributed algorithms typically assume strong memory consistency guarantees, but system implementations provide weaker guarantees for better performance and scalability. This motivates the study of how to implement programs designed for sequential consistency on platforms with weaker consistency models. Typically, such implementations are impossible using only read and write operations to shared variables. For example, Sparc's Total Store Order machines (and their even weaker Partial Store Order and Relaxed Memory Order machines), the PowerPC, Alpha, Java and most variants of processor consistency all require the use of strong and expensive built in hardware synchronization primitives to implement mutual exclusion. One variant of processor consistency originally proposed by Goodman and called here PC-G is an exception. It is known that PC-G provides just enough consistency to implement mutual exclusion using only reads and writes. This paper extends the study of Goodman's version of processor consistency from specific problems (such as mutual exclusion) to arbitrary ones. That is, we investigate the existence of compilers to convert programs that use shared read/write variables with sequentially consistent memory semantics, to programs that use read/write variables with PC-G consistency semantics.<p>We first provide a simple program transformation, and prove that it compiles any 2-process program with only single-writer variables from sequentially consistency to PC-G consistency. We next prove that no similar simple compiler exists for even a very restricted class of 2-process programs.<p>Even though our program transformation is not a general compiler for 3 or more processes, it does correctly transform some specific <i>n</i>-process programs from sequentially consistency to PC-G consistency. In particular, for the special case of the (necessarily randomized) Test&Set algorithm of Tromp and Vitanyi, our transformation extends to any number of processes. Thus, one notable outcome is an implementation of Test&Set on PC-G that uses only reads and writes of shared variables. This is the first expected, wait-free implementation of Test&Set on any weak memory model, and illustrates the use of randomization with a weak memory model.