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Scheduler Activations: Effective Kernel Support for User-Level Management of Parallelism

This paper explores user-level threads and kernel-level threads, and proposes a scheduler activation mechanism for efficient parallelism management without crossing the user/kernel boundary.

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Scheduler Activations: Effective Kernel Support for User-Level Management of Parallelism

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  1. CSE 60641: Operating Systems • Scheduler Activations: Effective Kernel Support for the User-Level Management of Parallelism. Thomas E. Anderson, Brian N. Bershad, Edward D. Lazowska, and Hank M. Levy, SOSP '91 • What is this paper trying to achieve? • User level threads • Kernel level threads • Scheduler activation CSE 60641: Operating Systems

  2. User level threads • Excellent performance (no crossing user/kernel boundary) • No protection issues because all threads belong to the same process • Cost of generality: user threads can be tuned to a particular application • Problems: • blocking call, kernel thread allocated to the user thread cannot be reclaimed • Allocate more virtual processors (unfair) • Kernel might interfere with critical sections • Processors cannot be returned CSE 60641: Operating Systems

  3. Solution: Scheduler activation • A mechanism for kernel and user level to cooperate • Kernel makes an upcall with a scheduler activation • You can either keep the activation, or perform the task informed by the activation. • User level -> kernel is still system call • Number of scheduler activations = number of virtual processors assigned to a process • Application is free to implement any scheduling policy using the assigned activations • Kernel will notify user if any thread blocks CSE 60641: Operating Systems

  4. Kernel upcalls because of • Add this processor • Processor has been preempted • Return to the ready list of the user-level thread that was executing in the context of the preempted activation • Scheduler activation has blocked • Blocked scheduler is no longer using its processor • Scheduler activation has unblocked • Ready to the ready list the user level thread that was executing in the context of the blocked activation. • New activation includes processor context the newly unblocked one and the one that was preempted CSE 60641: Operating Systems

  5. From user space to kernel • Add more processors • Processor is idle • Together, they can allow the kernel to give and take away processors dynamically CSE 60641: Operating Systems

  6. User level priority scheduling: • Deschedule a lower priorty thread • If the lower priority thread is in critical section, don’t do it • Detected using application maintained flags • Performance good: • Upcall latency: 5x kernel threads CSE 60641: Operating Systems

  7. Lessons learnt • Can kernel/user partnership make sense for other things? If so, how general can they be? CSE 60641: Operating Systems

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