Operating System - Note on Scheduler Activation

What is Scheduler Activation?

Scheduler Activation is a technique used to efficiently manage threads in a system that uses the many-to-many threading model — where multiple user-level threads are mapped to multiple kernel-level threads.

It aims to provide the flexibility of user-level threads (lightweight and fast switching) while retaining the benefits of kernel-level threads (true parallelism, blocking I/O handling).

 Why Is It Needed?

In systems where:

  • Threads are managed by a user-level library, the kernel is unaware of them.

  • The OS can’t directly manage scheduling or blocking of user threads.

This causes problems like:

  • If one thread blocks on I/O, the entire process may block.

  • The kernel can't preempt or schedule individual user threads.

Scheduler Activation bridges this gap.

 How Scheduler Activation Works

  1. The kernel provides "virtual processors" to the user-level thread library.

  2. Each virtual processor can run a user thread.

  3. When events like thread blocking, unblocking, or creation happen, the kernel notifies the user-level thread manager using upcalls.

  4. The user-level scheduler then adjusts its thread scheduling accordingly.

 Flow:

  • Thread blocks → Kernel sends upcall → User library knows and schedules another thread.

  • Thread unblocks → Kernel notifies → User scheduler wakes it up.

 Advantages

  • Supports parallelism on multi-core systems.

  • Enables efficient user-level scheduling.

  • Prevents entire process from blocking due to one thread.

 Disadvantages

  • More complex to implement.

  • Requires cooperation between kernel and user-space.

  • Can be tricky to handle race conditions and synchronization.