Scheduler Activation


This is a summary on scheduler activation. To discuss about scheduler activation, we must first understand what is a thread. A thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler.

Kernel Level Threads Pros/Cons

User Level Threads Pros/Cons

Some Problems about User-Level Threads on Kernel Interface

Scheduler Activation

The basic principle about scheduler activation is to expose revocation: telling me when you take something away. This is basically the same idea as the exokernel. For example, interfaces like

The basics about scheduler activation are

Scheduler Activation vs Kernel Threads

Key differences:

An important problem is what happened if a user thread is forced to be de-scheduler while it’s in a scheduler. The user thread will hold a a lock on user level run queue. That means no other user thread can be scheduled to run because none of them can acquire the lock. Because there’s no notion of “resume” in scheduling activation, we can’t really resume the execution in the scheduler. Thus, we run into a deadlock situation.

One solution is to detect whether we are using a lock and keep executing until we leave the locked region. Of course, there are too many gotchas in this solution.

Another solution is that the kernel can make a copy of the critical section and execute the critical section itself regardless of what the user thread chooses to do. Therefore, we can guarantee by the time you vector back into user space the lock is no longer held. So the kernel is basically executing the user code! Crazy, right? Now we ran into more gotchas. What if the code is written in Java? How to find a locked region in userspace? What if …

Another thing we want to mention is page fault. Page fault indicates that you are missing part of your address. So there will be a notification with a new scheduler activation. Once you do something with it, you will likely touch that same piece in the space and double fault again.

What is the solution?