Scheduler Activations: Effective Kernel Support for the User-Level Management of Parallelism

Threads are the vehicle,for concurrency in many approaches to parallel programming. Threads separate the notion of a sequential execution stream from the other aspects of traditional UNIX-like processes, such as address spaces and I/O descriptors. The objective of this separation is to make the expression and control of parallelism sufficiently cheap that the programmer or compiler can exploit even fine-grained parallelism with acceptable overhead. Threads can be supported either by the operating system kernel or by user-level library code in the application address space, but neither approach has been fully satisfactory. This paper addresses this dilemma. First, we argue that the performance of kernel threads is inherently worse than that of user-level threads, rather than this being an artifact of existing implementations; we thus argue that managing par- allelism at the user level is essential to high-performance parallel computing. Next, we argue that the lack of system integration exhibited by user-level threads is a consequence of the lack of kernel support for user-level threads provided by contemporary multiprocessor operating systems; we thus argue that kernel threads or processes, as currently conceived, are the wrong abstraction on which to support user- level management of parallelism. Finally, we describe the design, implementation, and performance of a new kernel interface and user-level thread package that together provide the same functionality as kernel threads without compromis- ing the performance and flexibility advantages of user-level management of parallelism.