Thread-based programs are inherently un-predictable due to involuntary context switching between threads and the blocking nature of concurrency primitives. It is very hard to reason about the response time, throughput and scalability of an application.
Edward A. Lee Professor and Chair of EE at UC Berkeley in his famous 2006 paper “The problem with threads” commented:
“They [threads] discard the most essential and appealing properties of sequential computation: understandability, predictability, and determinism.
Threads, as a model of computation, are wildly nondeterministic, and the job of the programmer becomes one of pruning that nondeterminism. …. Rather than pruning nondeterminism, we should build from essentially deterministic, composable components. Nondeterminism should be explicitly and judiciously introduced where needed, rather than removed where not needed. The consequences of this principle are profound. I argue for the development of concurrent coordination languages based on sound, composable formalisms. I believe that such languages will yield much more reliable, and more concurrent programs.”
AMPS provides one such ‘formalism’ as suggested by Lee. Event handlers written for AMPS are always:

• Understandable
• Predictable
• and above all Deterministic

By definition and design, an event handler in AMPS will terminate as soon as any operation comes that may require it to block. This requires a conscious effort on programmer’s part to remove all non-deterministic processes from the deterministic ones. This separation once done has long running consequences.