Threading is a Poor Concurrency Programming Model

Here’s why I try to avoid thread-based programming models for expressing concurrency:

The opponents of thread-based systems line up several drawbacks. For Ousterhout, who probably published the most well-known rant against threads [Ous96], the extreme difficulty of developing correct concurrent code–even for programming experts–is the most harmful trait of threads. As soon as a multi-threaded system shares a single state between multiple threads, coordination and synchronization becomes an imperative. Coordination and synchronization requires locking primitives, which in turn brings along additional issues. Erroneous locking introduces deadlocks or livelocks, and threatens the liveness of the application. Choosing the right locking granularity is also source of trouble. Too coarse locks slow down concurrent code and lead to degraded sequential execution. By contrast, too fine locks increase the danger of deadlocks/livelocks and increase locking overhead. Concurrent components based on threads and locks are not composable. Given two different components that are thread-safe, a composition of them is not thread-safe per se. For instance, placing circular dependencies between multi-threaded components unknowingly can introduce severe deadlocks.

Lee [Lee06] focuses on the lack of understandability and predictability of multi-threaded code, due to nondeterminism and preemptive scheduling. Multithreading appears to be error-prone, and very difficult to debug. The state explosion as a result from all possible interleavings of multiple threads renders a reasonable execution analysis of concurrent code virtually impossible. This is primarily caused by the unpredictability of preemptive scheduling. So, contrary to von Behren [vB03a], Lee argues that threads are precisely not a good abstraction for concurrent flows of execution. Quite the opposite, the oversimplifying abstraction of threads appears to be misleading, as it pretends a continuous execution of code that may not match any real runtime behavior.

Read the complete web page — it’s quite good.

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