Selecting Objective: Thread to Run
Scheduler selects a thread to run Context Switch Processor relinquished
SCHEDULING
Multiple Waiting Threads
A Single Running Thread
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Thread States: Initial View
Time-Sliced Scheduling
Each thread runs for a fixed amount of time. Threads are run in a round-robin sequence. Appropriate for regular multi-programming environments. Poor response time performance. Need better strategy for real-time systems.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Ready
Running
Thread waiting for scheduler to give it a turn.
Inactive
Thread unknown to scheduler.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Pending Threads
Threads must often wait for an event to occur or some shared resource to become available. Simply moving the task back to the ready state isn't appropriate because it may run again later only to discover that it must continue to wait, thus adding unnecessary task switching overhead.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Thread States: Revised View
Thread waiting for an external event or shared resource. Pending Running
Thread waiting for scheduler to give it control.
Ready
Inactive
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Non-Preemptive Context Switch
Yield Call State of Thread A Running Interrupt Routine Run-Time Kernel State of Thread B Interrupted Running Ready
Non-Preemptive Thread States
Pending Running
Context Switch Post Ready Ready Running
Yielding
Interrupted
Pending
Inactive
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Preemptive Context Switch
State of Thread A Running Interrupt Routine Run-Time Kernel Post State of Thread B Pending Ready Interrupted Running
Context Switch
Preemptive Thread States
Pending Running
Interrupted
Ready
Interrupted
Ready Inactive
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Priority-Based Scheduling
Each thread is assigned a priority number. Static priorities never change. Dynamic priorities can vary during execution.
Required to avoid Priority Inversion Problem.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Priority Inversion
Resource Owned by Low-Priority Thread High-Priority Thread Priority Inversion High-Priority Thread State Running Pending Running
Request resource
Run-Time Kernel
Release Resource
Low-Priority Thread State
Ready
Running
Ready
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Priority Inversion
Bounded Priority Inversion
Duration is no longer than that of the critical section where the lower-priority thread owns the resource.
Mars Pathfinder Suffered Unbounded Priority Inversion
Low-priority meteorological thread Acquired the (shared) bus. Woke up and preempted the meteorological thread. Woke up and was blocked because it couldn't acquire the bus; when it couldn't meet its deadline it reinitialized computer the via a hardware reset.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Medium-priority, long-running, communications thread High-priority bus management thread
Unbounded Priority Inversion
Occurs when a third (medium-priority) thread preempts the low-priority thread during the inversion for an indefinite amount of time.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Limiting the Duration of an Unbounded Priority Inversion
Objective:
To prevent low-priority thread from being preempted by medium-priority thread during the priority inversion.
Priority Inheritance Protocol
When a high-priority thread attempts to lock a mutex already locked by a lower-priority thread, the Priority Inheritance Protocol (PIP) temporarily raises the priority of the low-priority thread to match that of the blocked thread until the lowpriority thread unlocks the mutex. Advantage: It is transparent to the application. Disadvantage: Adds complexity to the kernel.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Strategies:
Priority Inheritance Protocol Priority Ceiling Protocol
Technique:
Manipulate thread priorities at run-time.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Priority Ceiling Protocol
Priority of the low-priority thread is raised immediately when it locks the mutex rather than waiting for a subsequent lock attempt by a higherpriority thread. Advantage: Easy to implement. Disadvantage: The priority ceiling value must be predetermined for use with the mutex; this value must be the highest among all the threads that attempt to lock the same mutex.
Copyright 2000, Daniel W. Lewis. All Rights Reserved.
Assigning Priorities
Scheduling: Threads with higher priority are scheduled to run first. Objective: Assign priorities in such a way that all outputs are computed before ...