1. A computer readable storage medium 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 runtime configured to allow the first task to be executed concurrently with a set of other tasks from the user code; providing the first task to a first scheduler specified by the user code with the runtime, the runtime maintaining a list of active schedulers that includes the first scheduler; and invoking the first task with the runtime responsive to the first scheduler.