C346 Pa3 W12 Src Common Base 503667
C346 Pa3 W12srccommonbasethreadjavac346 Pa3 W12srccommonbasethr
CLEANED: Consider a preemptive priority-scheduling algorithm based on dynamically changing priorities, where larger numbers indicate higher priority, and priorities change at rates α when waiting and β when running. When processes enter the ready queue with priority 0, answer questions about the resulting scheduling algorithm depending on the relationships between α and β, and whether they are positive or negative. Discuss a CPU scheduling algorithm that favors processes with least recent CPU usage, whether it favors I/O-bound processes, and if it causes starvation of CPU-bound processes. Explain potential issues with a variant of round robin scheduling that uses pointers to process control blocks (PCBs) and how malicious use could cause serious consequences. Describe a deadlock-free strategy for a variant of the dining philosopher’s problem where chopsticks are centrally placed and requests are made one at a time. Show that a system with shared resources is deadlock free if maximum needs per process are within certain bounds, and the sum of maximum needs is less than a specific value. Extend the classical dining philosophers problem with Java monitor synchronization, ensuring correctness, deadlock-freedom, and starvation-freedom with multiple philosophers and philosophers' ability to talk when not eating, using only Java synchronization primitives.
Paper For Above instruction
Introduction
Operating system scheduling algorithms govern how processes share the CPU, impacting system responsiveness and fairness. Dynamic priority scheduling introduces continual change in process priorities based on their CPU usage and waiting times, aiming to balance CPU-bound and I/O-bound processes. This paper analyzes a preemptive priority scheduling algorithm with dynamically changing priorities, evaluates scheduling strategies that favor certain process types, discusses potential vulnerabilities in PCB management, and explores deadlock avoidance in problems inspired by the dining philosophers problem. Furthermore, it extends the classical synchronization problem with Java's monitor constructs, presenting a comprehensive discussion on process scheduling, deadlock prevention, and concurrency control.
Analysis of Dynamic Priority Scheduling
The scheduling algorithm under consideration adjusts process priorities at rates α (when waiting in the ready queue) and β (when executing). When β > α > 0, the process’s priority increases more rapidly during execution than it decreases while waiting. This results in a scheduling policy that favors processes that have been recently active, effectively creating a form of aging that biases toward processes with higher execution time, leaning toward a form of preemptive priority scheduling that tends to favor recently or currently active processes. Processes that have been waiting longer may be overtaken in priority because as they wait, their priority diminishes at rate α, but processes currently executing or recently active have their priority boosted at rate β, which is higher.
Conversely, when α
Overall, when β > α > 0, the algorithm resembles a form of "aging" where active processes escalate in priority faster than they lose it, potentially leading to favoring processes that have been recently or currently running, possibly undermining fairness if certain processes continually preempt others. It tends toward a scheduling policy that emphasizes recent activity, possibly at the expense of long-waiting processes.
CPU Scheduling Favoring Least Recent CPU Usage
The proposed scheduling algorithm that favors processes with the least CPU time in a recent window (size τ) essentially implements a form of least-recently-used (LRU) strategy, favoring I/O-bound processes that tend to have short CPU bursts. I/O-bound processes often have minimal CPU usage before waiting for I/O operations, thus fitting into the recent CPU time window with lower aggregate usage and are prioritized to enhance response time and throughput.
Regarding starvation, this approach minimizes the risk for CPU-bound processes by giving preference to processes that have used less CPU recently. Since CPU-bound processes tend to have long, continuous CPU usage, they will not consistently have the least recent CPU time in the window, thus preventing indefinite postponement. As long as all processes have their maximum CPU time within the limits of the window size and the scheduling policy is fair, starvation is unlikely.
Thus, statement (a) is accurate because the algorithm favors I/O-bound processes by their shorter recent CPU time. Statement (b) holds because the recent-window-based approach aims to prevent starvation by continuously adjusting process priorities based on recent CPU usage, ensuring every process gets CPU time once it falls into the recent CPU window.
Round Robin with PCB Pointer Manipulation
In a variant of round robin scheduling where the ready queue contains pointers to process control blocks (PCBs), malicious users could exploit this to insert multiple pointers to their own process’s PCB. This can artificially inflate the process’s share of CPU time by allowing the scheduler to process the same process multiple times per cycle, effectively giving that process twice the execution opportunity compared to other processes.
The primary consequence of such manipulation is unfairness and potential violation of the process scheduling guarantees. It can cause a process to monopolize CPU time, leading to starvation of other processes. Furthermore, if the OS scheduler is unable to detect or verify pointer validity, the process can hijack system resources, cause data corruption, or even system crashes due to invalid pointer dereferences or inconsistent PCB states. This security loophole risks the stability and fairness of the entire system.
Hence, system integrity depends on protecting the atomicity and correctness of PCB pointers, and the OS must validate pointer authenticity to prevent malicious exploits.
Deadlock-Free Strategy in Modified Dining Philosophers
In a variant of the dining philosopher’s problem where all chopsticks are placed centrally, and any two can be used by a philosopher, deadlock can arise if everyone attempts to pick up their two required chopsticks simultaneously. To prevent deadlock, a simple rule can be:
- Request permission to pick up both chopsticks simultaneously: A philosopher can only acquire both chopsticks if they are both available at the same time. This can be implemented by a synchronized method in the monitor that checks if both chopsticks are free before granting permission. If not, the philosopher waits until both become available.
This rule ensures that philosophers do not pick up one chopstick at a time, which would otherwise allow for circular wait conditions. Only when both are available does the philosopher proceed, thus eliminating potential deadlock.
Implementation Sketch:
```java
public synchronized boolean requestChopsticks(int philosopherId) {
if (bothChopsticksFree(philosopherId)) {
// allocate chopsticks to philosopher
return true;
} else {
return false; // wait until both available
}
}
```
This approach guarantees that deadlock cannot occur because no philosopher holds a chopstick without acquiring the second, avoiding a cyclic wait.
Conditions for Deadlock-Free Resource Allocation
In a system with m resources of the same type shared by n processes, where requests and releases are for only one resource at a time, the classical deadlock avoidance condition is:
- The maximum number of resources each process may need is no more than m.
- The sum of all maximum needs across processes is less than m + n.
Justification:
If the total maximum resource demand is less than m + n, then at least one process can always acquire the remaining resources it needs to complete, thereby preventing any circular wait condition. This sum condition ensures that the system never reaches a state where all processes hold resources and wait indefinitely for others, as the total requests cannot reach a state of deadlock.
Extension of Dining Philosophers with Java Monitors
The classical dining philosophers problem can be extended with Java's monitor approach to include the additional constraint that only one philosopher can talk at a time when not eating, requiring synchronization that prevents both deadlocks and starvation.
Proposed Rules:
- Use a monitor object with synchronized methods for pickUp(), putDown(), requestTalk(), and endTalk().
- A philosopher requesting to eat must acquire both chopsticks atomically; if unavailable, they wait.
- To talk, a philosopher must first request permission via requestTalk(). If another philosopher is talking, they wait.
- When done, philosophers notify the monitor to release resources or end talking, waking up waiting philosophers.
Sample Implementation Principles:
```java
public class Monitor {
private boolean isTalking = false;
private int philosophersEating = 0;
private int availableChops;
// Initialize with number of chopsticks
public synchronized void pickUp(int id) {
while (!bothChopsticksAvailable(id)) {
wait();
}
// pick up chopsticks logic
}
public synchronized void putDown(int id) {
// release chopsticks
notifyAll();
}
public synchronized void requestTalk() {
while (isTalking) {
wait();
}
isTalking = true;
}
public synchronized void endTalk() {
isTalking = false;
notifyAll();
}
}
```
This design ensures mutual exclusion for talking, deadlock prevention, and fairness for process execution sequences, satisfying the assignment's correctness and fairness criteria.
Conclusion
Effective process scheduling and synchronization are essential to operating system stability. Dynamic priority scheduling seeks to balance responsiveness and fairness; mechanisms to prevent deadlock in resource allocation hinge on atomicity and request strategies; and enhanced synchronization constructs like Java monitors facilitate deadlock and starvation-free concurrency in complex problems like the dining philosophers. Implementing these strategies requires rigorous adherence to algorithmic rules which guarantee system correctness, fairness, and efficiency.
References
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