Podcast
Questions and Answers
What is the main difference between soft and hard real-time systems?
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.
In real-time CPU scheduling, event latency refers to the time it takes to service an event after it occurs.
True (A)
What are the two types of latencies that affect performance in real-time CPU scheduling?
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 ____________________.
In hard real-time systems, tasks must be serviced by their ____________________.
Match the following real-time CPU scheduling concepts with their definitions:
Match the following real-time CPU scheduling concepts with their definitions:
What is the conflict phase of dispatch latency?
What is the conflict phase of dispatch latency?
What is the period of process P1?
What is the period of process P1?
Process P2 has a CPU burst of 25.
Process P2 has a CPU burst of 25.
What is the time at which process P2 misses its deadline?
What is the time at which process P2 misses its deadline?
The POSIX.1b standard provides functions for managing ___________ threads.
The POSIX.1b standard provides functions for managing ___________ threads.
Match the POSIX scheduling classes with their descriptions:
Match the POSIX scheduling classes with their descriptions:
How many scheduling classes are defined by POSIX.1b for real-time threads?
How many scheduling classes are defined by POSIX.1b for real-time threads?
What is the main goal of releasing resources by low-priority processes?
What is the main goal of releasing resources by low-priority processes?
Rate Monotonic Scheduling guarantees hard real-time.
Rate Monotonic Scheduling guarantees hard real-time.
What is the rate of a periodic task?
What is the rate of a periodic task?
In Rate Monotonic Scheduling, a priority is assigned based on the ______________ of its period.
In Rate Monotonic Scheduling, a priority is assigned based on the ______________ of its period.
What is the CPU utilization of process P1 in the given example?
What is the CPU utilization of process P1 in the given example?
In Rate Monotonic Scheduling, shorter periods always result in lower priority.
In Rate Monotonic Scheduling, shorter periods always result in lower priority.
Match the following processes with their periods:
Match the following processes with their periods:
What is the primary characteristic of a periodic process?
What is the primary characteristic of a periodic process?
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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)
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