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:
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Threads are managed by a user-level library, the kernel is unaware of them.
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The OS can’t directly manage scheduling or blocking of user threads.
This causes problems like:
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If one thread blocks on I/O, the entire process may block.
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The kernel can't preempt or schedule individual user threads.
Scheduler Activation bridges this gap.
How Scheduler Activation Works
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The kernel provides "virtual processors" to the user-level thread library.
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Each virtual processor can run a user thread.
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When events like thread blocking, unblocking, or creation happen, the kernel notifies the user-level thread manager using upcalls.
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The user-level scheduler then adjusts its thread scheduling accordingly.
Flow:
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Thread blocks → Kernel sends upcall → User library knows and schedules another thread.
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Thread unblocks → Kernel notifies → User scheduler wakes it up.
Advantages
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Supports parallelism on multi-core systems.
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Enables efficient user-level scheduling.
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Prevents entire process from blocking due to one thread.
Disadvantages
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More complex to implement.
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Requires cooperation between kernel and user-space.
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Can be tricky to handle race conditions and synchronization.