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

2. Contents Problem Statement New Approach Scheduler Activation User-Level Threads Package Explicit Vectoring of Kernel Events to the User-Level Threads Scheduler Critical Section Performance Summary

3. Problem Statement The performance of kernel threads are inherently worse than that of user-level threads. – The program must cross extra protection boundary on kernel thread operation – With kernel threads management, a single implementation is imposed to all applications – With user-level threads management, application specific implementation can be easily done

4. TEST benchmark: Null Fork (the overhead of forking a thread) and Signal-Wait (the time for signal a waiting process and wait on a condition)

5. Why not use the user-level threads for the entire program?

6. What if a user-level thread makes a blocking IO call or has page faults?

7. What if the kernel preempts a process running a user level thread? The user level system cannot make scheduling decision

8. Effective user-level threads package that achieve the same functionality as kernel threads is on demand – To integrate user-level threads with other system services, kernel level support is required

9. Approach Provide each application with a virtual multiprocessor Each application knows how many processors are allocated to it Each application has complete control over which threads run on those processors The operating system has complete control over processor allocation among address space

10. The kernel notifies the user-level system of every kernel event that affects address space The user-level system notifies the kernel any events related to processor allocation

11. Scheduler Activations The kernel mechanism used to implement this approach The execution context for vectoring control from kernel to user-level threads system on kernel event – Notifies user-level threads system of kernel event by invoking a procedure in the user level threads library (upcall) The user-level system uses scheduler activation to handle user-level event such as execution of user- level threads and making requests of kernel Provides space in kernel to save a processor context

12. Explicit Vectoring of Kernel Events to the User-Level Thread Scheduler The communication between the kernel processor allocator and the user-level threads system is made in terms of scehduler Each scheduler activation has two execution stacks – One maps into the kernel – One maps into application address space When a program starts, the kernel creates a scheduler activation and assigns it to a processor

13. The kernel upcalls into the application at fixed entry point

15. What is user-level threads package? The thread package view each process as a virtual processor The virtual processors are multiplexed across the physical processors by the kernel User- level code in each virtual processor pulls threads off the ready list and runs them

17. Multiprocessor operating systems such as Mach, Topaz, and V provide kernel support for multiple threads per address space No processors are idle in the presence of ready list, if there is work to do No high-priority threads wait while a low- priority thread runs When a thread blocks, the processor the thread had been running on can be used to run another process

18. What if an activation’s user-level thread is stopped by kernel? – Once a user-level thread is stopped by kernel, the thread is not resumed by the kernel – A new scheduler activation is created – The kernel notifies the user system that the thread is blocked – The user-level system removes the state of the thread from the old activation and notifies the kernel – The user-level system decides which thread to run on the processor

19. Traditional kernel threads – When the kernel stops a user-level thread, it never notifies the user-level of the event – The kernel resumes the thread

23. Critical Section FastThreads uses unlocked per-processor free lists of thread control block – Access to the free lists must be done atomically Adopt a solution based on recovery  When a user-level thread is blocked, the user-level system checks whether it was executing in critical section  If so, the thread is continued via user-level context switch  When the thread exists, it hands the control over to the upcall via user context switch  The kernel places the thread onto the ready list

24. Implementation A prototype is built on the DES SRC FireFly multiprocessor workstation Moderate modification was applied to the kernel threads of Topaz, the operating system of FireFly – Modified Topaz to perform upcalls listed in Table II Straightforward modification was applied to FastThreads, the user-level system Default policy in FastThreads – Uses per-processor ready list

25. Last-in-first-out to improve cache locality

26. Performance enhancement – Discarded activation can be cached for future use – Discarded scheduler activations can be collected and returned to the kernel in bulk

29. Summary Processor allocation must be done by the kernel Thread scheduling must be done by each address space The kernel notifies the user-level thread scheduler of all the events that affect address space The user-level space notifies the kernel all the events that may affect the processor allocation decision