of Overview
CSE120
Principles Operating Systems
Prof Yuanyuan (YY) Zhou
Midterm Review
Oct 19th , 2010
The midterm
Architectural support for OSes
OS modules, interfaces, and structures
Processes
Threads
Synchronization
Scheduling
Deadlock
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Midterm
Arch Support for OSes
Covers material through scheduling
Based upon lecture material, optional
homeworks, and projects
No cheat sheets
Please, do not cheat
Types of architecture support
Manipulating privileged machine state
Generating and handling events
Events: Interrupts, exceptions, system calls, etc
Do not copy from your neighbor
No one involved will be happy, particularly the
teaching staff
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Privileged Instructions
What are privileged instructions?
Events
Who gets to execute them?
How does the CPU know whether they can be
executed?
Difference between user and kernel mode
Why do they need to be privileged?
What do they manipulate?
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OS services and abstractions
Interfaces
Operations supported by components
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What is the difference between exceptions and
interrupts?
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Modules
Modules
How do I/O devices use interrupts?
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OS Modules and Interfaces
Synchronous: fault (exceptions), system calls
Asynchronous: interrupts, software interrupt
What are faults, and how are they handled?
What are system calls, and how are they handled?
What are interrupts, and how are they handled?
Protected control registers
Memory management
I/O devices
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Events
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Processors
Memory
I/O
Secondary storage
Files
Protection
Account
Command interpreter (shell)
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Processes
Process Data Structures
What is a process?
What is the difference between a process
and a program?
What is contained in a process?
Process Control Blocks (PCBs)
State queues
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What information does it contain?
How is it used in a context switch?
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What are process states?
What is the process state graph?
When does a process change state?
How does the OS use queues to keep track of
processes?
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Process Manipulation
What does it mean for it to return twice?
What is a thread?
What does exec() on Unix do?
Threads
What does fork() on Unix do?
How is it different from fork?
How are fork and exec used to implement
shells?
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What is the difference between a thread and a
process?
How are they related?
Why are threads useful?
What is the difference between user-level
and kernel-level threads?
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What are the advantages/disadvantages of one
over another?
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Thread Implementation
How are threads managed by the run-time system?
Thread control blocks, thread queues
How is this different from process management?
Fork, yield, sleep, etc.
What does thread yield do?
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What guarantees do critical sections provide?
What are the requirements of critical sections?
Mutual exclusion (safety)
Progress (liveness)
Bounded waiting (no starvation: liveness)
Performance
What does Acquire do?
What does Release do?
What does it mean for Acquire/Release to be
atomic?
How can locks be implemented?
How does mutual exclusion relate to critical
sections?
What are the mechanisms for building critical
sections?
How does test-and-set/swap work?
What are the limitations of using spinlocks,
interrupts?
Locks, semaphores, monitors, condition variables
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Spinlocks
Disable/enable interrupts
Blocking (Nachos)
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Locks
What is mutual exclusion?
What is a critical section?
Global variables, static objects
Heap objects
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Mutual Exclusion
Race condition
Corruption
Bank account example
When are resources shared?
Voluntary and involuntary context switches
Coordinate access to shared data structures
Coordinate thread/process execution
What can happen to shared data structures if
synchronization is not used?
What is a context switch?
What is the difference between non-preemptive
scheduling and preemptive scheduling?
Why thread do we need synchronization?
What operations do threads support?
Synchronization
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What kind of lock does it implement?
Inefficient, interrupts turned off too long
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Semaphores
What is a semaphore?
Monitors
What does P/Decrement do?
What does V/Increment do?
How does a semaphore differ from a lock?
What is the difference between a binary semaphore and a
counting semaphore?
In what way does a monitor provide mutual exclusion?
How does a monitor differ from a semaphore?
How does a monitor differ from a lock?
What kind of support do monitors require?
Readers/Writers problem
Bounded Buffers problem
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Coordinating the execution of threads
Not mutual exclusion
What are the semantics of Wait?
What are the semantics of Signal?
What are the semantics of Broadcast?
How are condition variables different from
semaphores?
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Shared data
Procedures
A lock for mutual exclusion to procedures (w/ a queue)
Queues for the condition variables
What is the difference between Hoare and Mesa
monitors?
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What does the implementation of a monitor look
like?
Operations
Language, run-time support
Implementing Monitors
What is a condition variable used for?
To what extent is it provided?
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Condition Variables
Shared data
Procedures
Synchronization
When do threads block on semaphores?
When are they woken up again?
Using semaphores to solve synchronization
problems
What is a monitor?
Semantics of signal (whether the woken up waiter gets to
run immediately or not)
What are their tradeoffs?
What does Java provide?
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Locks and Condition Vars
Scheduling
In Nachos, we dont have monitors
But we want to be able to use condition variables
So we isolate condition variables and make them
independent (not associated with a monitor)
Instead, we have to associate them with a lock
(mutex)
Now, to use a condition variable
Components
When does scheduling happen?
Scheduler (dispatcher)
Job changes state (e.g., waiting to running)
Interrupt, exception
Job creation, termination
Threads must first acquire the lock (mutex)
CV::Wait releases the lock before blocking, acquires it after
waking up
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Scheduling Goals
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Starvation
Goals
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Maximize CPU utilization
Maximize job throughput
Minimize turnaround time
Minimize waiting time
Minimize response time
Starvation
Causes
Indefinite denial of a resource (CPU, lock)
Side effect of scheduling
Side effect of synchronization
Operating systems try to prevent starvation
What is the goal of a batch system?
What is the goal of an interactive system?
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Scheduling Algorithms
What are the properties, advantages and
disadvantages of the following scheduling
algorithms?
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Deadlock
First Come First Serve (FCFS)/First In First Out
(FIFO)
Shortest Job First (SJF) /shortest remaining time
first
Priority
Round Robin
Multilevel feedback queues
What scheduling algorithm does Unix use?
Why?
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int x = 0;
int i, j;
Ignore it
Prevent it (prevent one of the four conditions)
Avoid it (have tight control over resource
allocation)
Detect and recover from it
void AddToX() {
for (i = 0; i < 100; i++) x++;
}
void SubFromX() {
for (j = 0; j < 100; j++) x--;
}
Which of the four approaches above does it
implement?
How does it compute whether an allocation is
safe?
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A fun problem: Race Conditions
What is the Bankers algorithm?
Resource allocation graph (RAG)
Waits for graph (WFG)
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Dealing with deadlock
Mutual exclusion
Hold and wait
No preemption
Circular wait
How to visualize, represent abstractly?
Deadlock Approaches
Deadlock happens when processes are
waiting on each other and cannot make
progress
What are the conditions for deadlock?
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One thread does addition, and the other
thread does subtraction
What is the range of possible values for x?
Why?
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Good Luck
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