Scheduler activations are a threading mechanism that, when implemented in an operating system's process scheduler, provide kernel-level thread functionality with user-level thread flexibility and performance. This mechanism uses a so-called "N:M" strategy that maps some N number of application threads onto some M number of kernel entities, or "virtual processors." This is a compromise between kernel-level ("1:1") and user-level ("N:1") threading. In general, "N:M" threading systems are more complex to implement than either kernel or user threads, because both changes to kernel and user-space code are required.
Scheduler activations were proposed by Anderson, Bershad, Lazowska, and Levy in Scheduler Activations: Effective Kernel Support for the User-Level Management of Parallelism in 1991. Support was implemented in the NetBSD kernel by Nathan Williams but has since been abandoned in favor of 1:1 threading. FreeBSD had a similar threading implementation called Kernel Scheduled Entities which is also being retired in favor of 1:1 threading. Scheduler activations were also implemented as a patch for the Linux kernel by Vincent Danjean: Linux Activations, the user-level part being done in the Marcel thread library.
References
- Anderson, Thomas E.; Bershad, Brian N.; Lazowska, Edward D.; Levy, Henry M. (September 1991). "Scheduler activations: Effective kernel support for the user-level management of parallelism". Proceedings of the thirteenth ACM symposium on Operating systems principles. pp. 95–109. doi:10.1145/121132.121151. ISBN 0897914473. S2CID 264864317.
- An Implementation of Scheduler Activations on the NetBSD Operating System
- Significant changes from NetBSD 4.0 to 5.0
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