Scheduler Activations: A New Approach to Parallelism

1. Summarizing presentation ofScheduler Activations–A different approach to parallelism Jonas Johansson

2. Contents • Introduction • The Problem • The Approach • Result • Legacy • Summary • Conclusions and comments

3. Introduction – About the paper • Written in 1991 • Authors: • Anderson, Bershad, Lazowska, Levy • University of Washington (Seattle) • Summary • Describes drawbacks to the management of parallelism (the way it was back in 1991) • Suggests a new solution (”scheduler activations”) • Compares performance (old methods, scheduler activations)

4. Introduction – Ambition of this presentation • Give an understanding of: • the characteristics of kernel threads and user-level threads • the scheduler activations approach • Tie this to topics covered in this course • Parallelism • Lock holding threads (e.g. mutexlock) • I/O, blocking

5. Introduction - Threads review Threads • Several threads in each process • Parallelism • Share address space, system resources (e.g. files and terminals) etc. • Less overhead when exchanging data between threads compared to inter-process communication (due to shared memory etc) • Two types of threads: • Kernel threads • User-level threads Address space Files Code Multithreaded process

6. Introduction – Kernel and User-level threads User level Applications Kernel level Kernel System resources CPU Memory Devices

7. Introduction – Kernel threads Application User level Kernel level The kernel will create the threads The kernel is also responsible for thread scheduling KERNEL CPU1 CPU2 System resources

8. Introduction – Kernel threads Kernel threads • Supported directly by the Operating system • The kernel handles: • Thread creation • Thread scheduling • Thread management

9. Introduction – Kernel threads

10. Introduction – User-level threads Thread library Application User level Threads are created at user level Thread scheduling is done at user level The user threads are mapped onto kernel threads (here 1 thread/processor “User-level threads are built on top of kernel threads” Kernel level CPU1 CPU2 System resources

11. Introduction – User-level threads User-level threads • Implemented by a thread library at user level • Creation, scheduling and management of threads without support from the kernel • Creation and management is fast • Built on top of kernel threads

12. Introduction – User-level threads

13. Introduction – Simplified Kernel threads ”work right” but perform poorly, user-level threads perform well but doesn’t always “work right”

14. Problem

15. Problem – in brief ”Threads can be supported either at user level or in the kernel. Neither approach has been fully satisfactory.”

16. Problem – Simplified summary • 2 main problems in traditional thread systems • The user-level is not informed of kernel events such as I/O blocking • Processor lost during the block • Kernel threads are scheduled without regard to the user-level thread state • Lock-holding threads can be de-scheduled In other words: Communication between the kernel and the user level is not good enough…

17. Approach

18. Approach – Scheduler activations • ”Scheduler activations” • A newly designed kernel interface • A new user-level thread system Note that this is still a user-level approach but with a new kind of kernel support Scheduler activations ≈ User-level thread system with better communication and interaction with the kernel

19. Approach – Whatdoeswhat?

20. Approach • Why called Scheduler activation? • The scheduler is activated when it needs to make a decision about scheduling • The key point is communication between the kernel and the user level

21. Approach – Illustration of startup • An application is started • The kernel creates a scheduler activation and assigns this to a processor • The same processor is used to start running the thread • When the program runs more threads will be created • More threads than processors ->A new scheduler activation is created… Application User level VCPU1 Memory space Kernel level KERNEL VCPU2 CPU1 CPU2

22. Approach – Illustration of communication WantmoreCPUs Application Application User level ! This processor is idle VCPU2 VCPU1 VCPU1 VCPU2 VCPU3 Memory space Memory space This processor has been preempted Add this processor KERNEL Kernel level CPU1 CPU2 CPU3 CPU4 CPU5

23. Problem – Simplified summary • Main problems in traditional thread systems • The user-level is not informed of kernel events such as I/O blocking • Processor lost during the block • Kernel threads are scheduled without regard to the user-level thread state • Lock-holding threads can be descheduled In other words: Communication between the kernel and the user level is not good enough…

24. Approach – Blocking • With user-level threads • When a user-level thread blocks, its kernel thread also blocks • The physical processor is lost during the block • With scheduler activations • The user-level is informed about the block and can therefore run another thread on the processor • When unblocked, the user-level is informed and can choose which thread to schedule next

25. Problem – Simplified summary • Main problems in traditional thread systems • The user-level is not informed of kernel events such as I/O blocking • Processor lost during the block • Kernel threads are scheduled without regard to the user-level thread state • Lock-holding threads can be de-scheduled In other words: Communication between the kernel and the user level is not good enough…

26. Approach – Scheduling • With user-level threads • Timeslicing of kernel threads • Processor idle while waiting for a lock holding thread that has been de-scheduled • With scheduler activations • The user level: • Knows when a thread is holding a lock • Is responsible for all of the scheduling Will not de-schedule lock holders

27. Result

28. Result – Effects of scheduleractivations • Performance • Functionality • Flexibility

29. Result – Measuring performance • Performance • Measured by running a parallel application with • Little use of kernel services (100% memory) • Much use of kernel services (limited memory) • Blocking… • Two simultaneous applications (100% memory) • Lock-holding threads…

30. Result – Performance – Littleuse of kernel services • Almost no I/O • Speedup compared to sequential implementation • Scheduler activationsperform as good as user-level threads Topaz = Kernel threads orig = User-level threads new = Scheduler activations

31. Result – Performance – Muchuse of kernel services • Much use of I/O • Working set stops fitting into memory at about 50% -> kernel services • Scheduler activations and kernel threads • Handle I/O in a good way • User- level threads • Physical processor is lost during I/O blocks Topaz = Kernel threads orig = User-level threads new = Scheduler activations

32. Result – Performance – Muchuse of kernel services • Simultaneous applications • Two copies of the same program • This will also generate kernel events (system-induced events) • Performance difference • User-level – Time slicing gives idling processors • Kernel level – ”Expensive” thread operations Thread system Speed-up Kernel threads: 1.29 User-level threads: 1.26 Scheduler activations: 2.45

33. Result – Effects of scheduleractivations • Performance • Functionality • Flexibility • Pass

34. Result – Functionality • Functionality • Same as kernel threads (even with I/O, page faults, multiprogramming) • No idle processor when there are ready threads • When a thread blocks, the processor that was running that thread must be able to run another thread during the block Pass Pass

35. Result – Effects of scheduleractivations • Performance • Functionality • Flexibility • Pass • Pass

36. Result – Flexibility • Goal: Simple application-specific customization • The kernel is unaware of the scheduling policies • All scheduling is decided at user level (by the programmer) • The programmercanmodify the policy for schedulingdecisionsrelativelyeasily • Pass

37. Result – Effects of scheduleractivations • Performance • Functionality • Flexibility • Pass • Pass • Pass

38. Results – Summary • Goals were met • Performance • Equal or better than user-level thread performance • Great performance increase when dealing with I/O • Functionality • Equal to that of kernel threads • Flexibility • Programming an application using scheduler activations is similar to traditional multiprogramming • However…

39. Legacy • To implement there is a need of modifying both user space code and the kernel – Hard! • Implemented in • NetBSD kernel by Nathan Williams • FreeBSD - KSE project (threading system similar to Scheduler activations) • Later replaced with kernel threads • Scheduler activations has been implemented mostly for research purposes

40. Summary

41. Conclusion and comments • Suffers from none of the weaknesses with user-level and kernel level threads • Logical approach – ”makes sense” • Complex implementation still makes it worse than the traditional approaches • Implementation: Though important, only briefly covered in the paper

42. Summarizing presentation ofScheduler Activations–A different approach to parallelism Jonas Johansson