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.

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