The Sequential Prison

摘要

We are trapped in a sequential prison. We use sequential character strings to write sequential programs to control sequential computers. No wonder concurrency remains elusive. How did we come to be here? The high cost of vacuum tube logic forced sequence upon early computer builders. Sequential character strings were the economic way to describe what sequential computers should do. Sequential programs controlled the expensive part of the machine, namely logic. The lethargic pace of logic circuits masked the cost of moving data over distance, allowing programming languages to ignore the cost of communication. Today, the time delay and energy cost of communicating over distance dominate modern computers; logic is essentially free. Why then, do programming languages continue to control logic and largely ignore communication? It will take a broad effort to escape our sequential prison, requiring changes in hardware, programming notations and the ways in which they are expressed. Most importantly, it will require recognizing that we are in sequential prison, and planning for an escape.