Scheduler activations
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Scheduler activations allow for fast user-level thread management while also enabling real thread parallelism on multiprocessors. With scheduler activations, the kernel scheduler uses upcalls to notify the user-level scheduler of scheduling events like new, preempted, blocked, and unblocked threads. This allows the user-level scheduler to maintain thread parallelism by scheduling another user-level thread when one blocks. The number of activations, which approximate kernel threads, can equal the number of CPUs.
![Scheduler Activations
• First proposed by [Anderson et al. 91]
• Idea: Both schedulers co-operate
• User scheduler uses system calls
• Kernel scheduler uses upcalls!
• Two important concepts
– Upcalls
• Notify the user-level of kernel scheduling events
– Activations
• A new structure to support upcalls and execution
– approximately a kernel thread
• As many running activations as (allocated) processors
• Kernel controls activation creation and destruction](https://image.slidesharecdn.com/scheduleractivations-110602081914-phpapp01/85/Scheduler-activations-8-320.jpg)
Scheduler activations
- 1. Scheduler Activations With some slides modified from Raymond Namyst, U. Bordeaux
- 2. User-level Threads User Mode Scheduler Scheduler Scheduler Process A Process B Process C Scheduler Kernel Mode
- 3. User-level Threads Fast thread management (creation, deletion, switching, synchronisation…) Blocking blocks all threads in a process – Syscalls – Page faults No thread-level parallelism on multiprocessor
- 4. Kernel-Level Threads User Mode Process A Process B Process C Scheduler Kernel Mode
- 5. Kernel-level Threads Slow thread management (creation, deletion, switching, synchronisation…) • System calls Blocking blocks only the appropriate thread in a process Thread-level parallelism on multiprocessor
- 6. Hybrid Multithreading User Mode Scheduler Scheduler Scheduler Process A Process B Process C Scheduler Kernel Mode
- 7. Hybrid Multithreading Can get real thread parallelism on multiprocessor Blocking still a problem!!!
- 8. Scheduler Activations • First proposed by [Anderson et al. 91] • Idea: Both schedulers co-operate • User scheduler uses system calls • Kernel scheduler uses upcalls! • Two important concepts – Upcalls • Notify the user-level of kernel scheduling events – Activations • A new structure to support upcalls and execution – approximately a kernel thread • As many running activations as (allocated) processors • Kernel controls activation creation and destruction
- 9. Scheduler Activations • Instead of CPU time wasted syscall User Space I/O request interrupt Kernel Space Hardware • …rather use the following scheme: CPU used User Space upcall upcall Kernel Space Hardware
- 10. Upcalls to User-level scheduler • New – Allocated a new virtual CPU – Can schedule a user-level thread • Preempted – Deallocated a virtual CPU – Can schedule one less thread • Blocked – Notifies thread has blocked – Can schedule another user-level thread • Unblocked – Notifies a thread has become runnable – Must decided to continue current or unblocked thread
- 11. Working principle • Blocking syscall scenario on 2 processors Process User scheduler
- 12. Working principle • Blocking syscall scenario on 2 processors Process new
- 13. Working principle • Blocking syscall scenario on 2 processors Process new
- 14. Working principle • Blocking syscall scenario on 2 processors Process
- 15. Working principle • Blocking syscall scenario on 2 processors Process preempt Preempt
- 16. Working principle • Blocking syscall scenario on 2 processors Process
- 17. Working principle • Blocking syscall scenario on 2 processors Process Blocking syscall
- 18. Working principle • Blocking syscall scenario on 2 processors Process New + blocked
- 19. Working principle • Blocking syscall scenario on 2 processors Process I/O completion
- 20. Working principle • Blocking syscall scenario on 2 processors Process Unblocked
- 21. Working principle • Blocking syscall scenario on 2 processors Process
- 22. Scheduler Activations • Thread management at user-level – Fast • Real thread parallelism via activations – Number of activations (virtual CPU) can equal CPUs • Blocking (syscall or page fault) creates new activation – User-level scheduler can pick new runnable thread. • Fewer stacks in kernel – Blocked activations + number of virtual CPUs
- 23. Adoption • Adopters – BSD “Kernel Scheduled Entities” – K42 – Digital UNIX – Solaris – Mach • Linux -> kernel threads