Gregory M. Papadopoulos

Monsoon: an explicit token-store architecture

Dataflow architectures tolerate long unpredictable communication delays and support generation and coordination of parallel activities directly in hardware, rather than assuming that program mapping will cause these issues to disappear. However, the proposed mechanisms are complex and introduce new mapping complications. This paper presents a greatly simplified approach to dataflow execution, called the explicit token store (ETS) architecture, and its current realization in Monsoon. The essence of dynamic dataflow execution is captured by a simple transition on state bits associated with storage local to a processor. Low-level storage management is performed by the compiler in assigning nodes to slots in an activation frame, rather than dynamically in hardware. The processor is simple, highly pipelined, and quite general. It may be viewed as a generalization of a fairly primitive von Neumann architecture. Although the addressing capability is restrictive, there is exactly one instruction executed for each action on the dataflow graph. Thus, the machine oriented ETS model provides new understanding of the merits and the real cost of direct execution of dataflow graphs.

T: a multithreaded massively parallel architecture

What should the architecture of each node in a general purpose, massively parallel architecture (MPA) be? We frame the question in concrete terms by describing two fundamental problems that must be solved well in any general purpose MPA. From this, we systematically develop the required logical organization of an MPA node, and present some details of *T (pronounced Start, a concrete architecture designed to these requirements. *T is a direct descendant of dynamic dataflow architectures, and unifies them with von Neumann architectures. We discuss a hand-compiled example and some compilation issues.

Multithreading: a revisionist view of dataflow architectures

Although they are powerful intermediate representations for compilers, pure dataflow graphs are incomplete, and perhaps even undesirable, machine languages. They are incomplete because it is hard to encode critical sections and imperative operations which are essential for the efficient execution of operating system functions, such as resource management. They may be undesirable because they imply a uniform dynamic scheduling policy for all instructions, preventing a compiler from expressing a static schedule which could result in greater run time efficiency, both by reducing redundant operand synchronization, and by using high speed registers to communicate state between instructions. In this paper, we develop a new machine-level programming model which builds upon two previous improvements to the dataflow execution model: sequential scheduling of instructions, and multiported registers for expression temporaries. Surprisingly, these improvements have required almost no architectural changes to explicit token store (ETS) dataflow hardware, only a shift in mindset when reasoning about how that hardware works. Rather than viewing computational progress as the consumption of tokens and the firing of enabled instructions, we instead reason about the evolution of multiple, interacting sequential threads, where forking and joining are extremely efficient. Because this new paradigm has proven so valuable in coding resource management operations and in improving code efficiency, it is now the cornerstone of the Monsoon instruction set architecture and macro assembly language. In retrospect, this suggests that there is a continuum of multithreaded architectures, with pure ETS dataflow and single threaded von Neumann at the extrema. We use this new perspective to better understand the relative strengths and weaknesses of the Monsoon implement ation.

The explicit token store

This paper presents an unusually simple approach to dynamic dataflow execution, called the Explicit Token Store (ETS) architecture, and its current realization in Monsoon. The essence of dynamic dataflow execution is captured by a simple transition on state bits associated with storage local to a processor. Low-level storage management is performed by the compiler in assigning nodes to slots in an activation frame, rather than dynamically in hardware. The processor is simple, highly pipelined, and quite general. There is exactly one instruction executed for each action on the dataflow graph. Thus, the machine-oriented ETS model provides new insight into the real cost of direct execution of dataflow graphs.

Monsoon: a dataflow computing architecture suitable for intelligent control

Intelligent control is the fusion of artificial intelligence, conventional control theory, and operations research. The Monsoon dataflow multiprocessor is a parallel, general-purpose computer which is under construction at MIT in cooperation with Motorola Inc. The Monsoon system is briefly described, and a simple extension to the Monsoon token queues is proposed to permit preemptive scheduling of time-critical tasks and to make the system suitable as an intelligent control execution vehicle.<<ETX>>