Runtocompletion

Runtocompletion is a design principle in concurrent systems in which a unit of work is processed from start to finish without preemption or interleaving by other work items. The term is frequently described as run-to-completion (RTC), and it characterizes a non-preemptive handling model where a worker, thread, or actor completes the current task before taking on the next.

In practice, runtocompletion is most closely associated with actor-based and event-driven architectures. For example, in actor models such as those found in Erlang or Akka, an actor processes one message at a time, and the processing of that message runs to completion before the actor begins handling the next message. This provides strong locality of reference and simplifies reasoning about state and side effects within a single unit of work.

Benefits of runtocompletion include easier reasoning about correctness, reduced risk of certain race conditions, and deterministic behavior within the scope of an individual task. However, it can introduce latency and reduce throughput if some tasks are long-running, since other work items must wait their turn. This head-of-line blocking is a common critique of RTC systems.

Implementation considerations include using a single-threaded or partitioned execution model to ensure a unit runs to completion, or structuring long tasks to yield control and perform work in smaller chunks. In many systems, runtocompletion is balanced with asynchronous I/O, task splitting, or background processing to mitigate latency while preserving the simplicity and predictability of RTC semantics.

Related concepts include cooperative multitasking and preemptive scheduling. RTC remains a foundational idea in designing predictable, non-interleaving processing in concurrent architectures.