Operating System Concepts - 10th Edition Chapter 5

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Questions and Answers

What is the main difference between soft and hard real-time systems?

Soft real-time systems are used for general-purpose computing, while hard real-time systems are used for embedded systems.

Soft real-time systems have flexible deadlines, while hard real-time systems have strict deadlines. (correct)

Soft real-time systems prioritize tasks based on priority, while hard real-time systems prioritize tasks based on deadlines.

Soft real-time systems have strict deadlines, while hard real-time systems have flexible deadlines.

In real-time CPU scheduling, event latency refers to the time it takes to service an event after it occurs.

True

What are the two types of latencies that affect performance in real-time CPU scheduling?

Interrupt latency and dispatch latency

In hard real-time systems, tasks must be serviced by their ____________________.

deadlines Signup and view all the answers

Match the following real-time CPU scheduling concepts with their definitions:

Event Latency = Time from event occurrence to service Interrupt Latency = Time from interrupt arrival to start of routine Dispatch Latency = Time for scheduler to switch processes Signup and view all the answers

What is the conflict phase of dispatch latency?

Preemption of any process running in kernel mode Signup and view all the answers

What is the period of process P1?

50 Signup and view all the answers

Process P2 has a CPU burst of 25.

False Signup and view all the answers

What is the time at which process P2 misses its deadline?

80 Signup and view all the answers

The POSIX.1b standard provides functions for managing ___________ threads.

real-time Signup and view all the answers

Match the POSIX scheduling classes with their descriptions:

SCHED_FIFO = FCFS strategy with a FIFO queue SCHED_RR = Time-slicing for threads of equal priority Signup and view all the answers

How many scheduling classes are defined by POSIX.1b for real-time threads?

2 Signup and view all the answers

What is the main goal of releasing resources by low-priority processes?

To give priority to high-priority processes Signup and view all the answers

Rate Monotonic Scheduling guarantees hard real-time.

False Signup and view all the answers

What is the rate of a periodic task?

1/p Signup and view all the answers

In Rate Monotonic Scheduling, a priority is assigned based on the ______________ of its period.

inverse Signup and view all the answers

What is the CPU utilization of process P1 in the given example?

0.40 Signup and view all the answers

In Rate Monotonic Scheduling, shorter periods always result in lower priority.

False Signup and view all the answers

Match the following processes with their periods:

P1 = 50 P2 = 100 Signup and view all the answers

What is the primary characteristic of a periodic process?

Requires CPU at constant intervals Signup and view all the answers

Study Notes

Real-Time CPU Scheduling

Can present obvious challenges
Soft real-time systems: critical real-time tasks have the highest priority, but no guarantee as to when tasks will be scheduled
Hard real-time systems: tasks must be serviced by their deadlines

Event Latency

The amount of time that elapses from when an event occurs to when it is serviced
Two types of latencies affect performance:
- Interrupt latency: time from arrival of interrupt to start of routine that services interrupt
- Dispatch latency: time for schedule to take current process off CPU and switch to another

Dispatch Latency

Conflict phase of dispatch latency:
- Preemption of any process running in kernel mode
- Release by low-priority process of resources needed by high-priority processes

Priority-based Scheduling

For real-time scheduling, scheduler must support preemptive, priority-based scheduling
Only guarantees soft real-time, not hard real-time
Processes have new characteristics: periodic ones require CPU at constant intervals
Has processing time t, deadline d, period p, where 0 ≤ t ≤ d ≤ p
Rate of periodic task is 1/p

Rate Monotonic Scheduling

A priority is assigned based on the inverse of its period
Shorter periods = higher priority; longer periods = lower priority
CPU utilization of a process Pi is the ratio of its burst to its period — ti / pi

POSIX Real-Time Scheduling

The POSIX.1b standard provides functions for managing real-time threads
Defines two scheduling classes for real-time threads:
- SCHED_FIFO: threads are scheduled using a FCFS strategy with a FIFO queue; no time-slicing for threads of equal priority
- SCHED_RR: similar to SCHED_FIFO except time-slicing occurs for threads of equal priority
Defines two functions for getting and setting scheduling policy:
- pthread_attr_getsched_policy(pthread_attr_t *attr, int *policy)
- pthread_attr_setschedpolicy(pthread_attr_t *attr, int policy)