1. A computer-readable storage medium, which does not include transitory propagating signals, storing computer-executable instructions that, when executed by at least one computer system, perform a method comprising: initializing a first task from user code for execution with a parallel execution runtime configured to allow the first task to be executed concurrently with a set of other tasks from the user code in a runtime environment; maintaining a task status corresponding to the first task with the parallel execution runtime; providing the first task to a first plug-in scheduler specified by the user code with the parallel execution runtime, the parallel execution runtime maintaining a list of active schedulers that includes the first plug-in scheduler; and invoking the first task with the parallel execution runtime responsive to the first plug-in scheduler.