Operating Systems: Processes, Memory, Scheduling and Deadlocks

1. What Is an Operating System?

An operating system coordinates the use of hardware such as the processor, memory, storage, displays, keyboards, and network devices. It also provides services that applications use, including file access, memory allocation, process creation, networking, and security checks.

The OS acts as both a resource manager and an abstraction layer. For example, an application works with a file name instead of directly controlling storage blocks on a disk.

2. Types of Operating Systems

Operating-system categories can overlap. For example, one OS may be multitasking, multi-user, networked, and capable of real-time tasks in different environments.

Hard and soft real-time systems

• Hard real-time: missing a deadline may cause system failure or serious harm.
• Soft real-time: deadlines are important, but occasional lateness reduces quality rather than causing total failure.

3. Processes, Threads, and Process States

Common process states

A running process may move to the waiting state when it requests I/O. When the required event occurs, it returns to the ready state. A scheduler can also preempt a running process and return it to the ready state.

Process Control Block (PCB)

The OS keeps process-related information in a data structure commonly called a Process Control Block. Exact contents vary by operating system, but a PCB commonly stores:

• Process identifier and current state
• Program counter and CPU-register information
• Scheduling priority and queue information
• Memory-management information
• Open files, I/O status, and accounting information
• Security or credential information

Switching the CPU from one process or thread to another is called a context switch. It enables multitasking but has overhead because the OS must save and restore execution state.

4. CPU Scheduling

CPU scheduling selects a ready process or thread to execute. Scheduling policies attempt to balance responsiveness, fairness, throughput, and efficient processor use.

Common scheduling measures

• CPU utilization: percentage of time the CPU is busy.
• Throughput: number of completed jobs in a time interval.
• Turnaround time: completion time minus arrival time.
• Waiting time: total time spent waiting in the ready queue.
• Response time: time from arrival until the process first receives CPU service.

5. Memory Management and Virtual Memory

The OS manages physical memory and gives processes protected virtual address spaces. The familiar register-cache-RAM-storage hierarchy is primarily a hardware-storage hierarchy; the OS mainly manages RAM, virtual memory, and storage-backed data.

Memory-allocation approaches

Paging and page faults

During paging, the OS and memory-management hardware translate virtual addresses to physical addresses. A cache of recent address translations is commonly called a Translation Lookaside Buffer (TLB).

A page fault occurs when a process accesses a page that is not currently in physical memory. The OS may load the required page from storage, update mappings, and resume the process. A page fault is a normal virtual-memory event, not automatically a program error.

Excessive paging activity can lead to thrashing, where the system spends much of its time moving pages between storage and memory instead of performing useful work.

6. Synchronization and Deadlocks

Critical sections and synchronization

When concurrent processes or threads share data, they can create race conditions unless access is coordinated. A critical section is code that accesses shared data and must not be executed unsafely by multiple tasks at the same time.

• Mutex: a lock that allows one task at a time to enter a protected section.
• Semaphore: a synchronization counter used to coordinate access to limited resources or events.
• Monitor: a higher-level synchronization construct that combines shared data with controlled operations.

Deadlock

A deadlock occurs when a group of processes or threads waits indefinitely because each one holds a resource and waits for a resource held by another member of the group.

Four necessary deadlock conditions

1. Mutual exclusion: at least one resource is non-shareable.
2. Hold and wait: a process holds resources while waiting for additional ones.
3. No preemption: resources cannot be forcibly removed in the normal case.
4. Circular wait: a circular chain exists in which each process waits for a resource held by the next process.

7. File Systems and Storage Management

File-system concepts

• File: a named collection of related data.
• Directory: a structure that maps names to files or other directories.
• Metadata: information about a file, such as size, ownership, permissions, timestamps, and location details.
• File permissions: rules that control who can read, write, execute, or modify a file.

File-allocation methods

Disk scheduling

Disk-scheduling algorithms are most relevant to rotating hard disks because they have mechanical seek time and rotational delay. Solid-state drives do not have moving heads, so their performance concerns and scheduling behavior differ.

• FCFS: serves requests in arrival order.
• SSTF: serves the request with the shortest seek distance; some requests may wait too long.
• SCAN: moves the disk arm in one direction while serving requests, then reverses.
• C-SCAN: serves requests in one direction and returns to the beginning without serving requests on the return trip.
• LOOK and C-LOOK: reverse or return at the last pending request rather than moving to the physical end of the disk.

8. Kernel, System Calls, and Interrupts

Kernel design styles

• Monolithic kernel: many core services run in kernel space.
• Microkernel: keeps a smaller core in kernel space and moves more services into user space.
• Hybrid design: combines ideas from more than one approach.

9. Examples of Operating-System Families

The following are examples of operating-system families and platforms, not a ranking. Features vary across editions, versions, device manufacturers, and system configurations.

10. Quick Revision and Practice Questions

Practice questions

1. What is the difference between a program and a process?
   Answer: A program is passive code stored on disk; a process is that program while it is executing with allocated resources and state.
2. What is the main difference between a process and a thread?
   Answer: A process has its own address space and resources, while threads in the same process usually share that address space and resources.
3. Why can Round Robin be inefficient when the time quantum is too small?
   Answer: Too many context switches occur, increasing overhead.
4. Name the four necessary conditions for deadlock.
   Answer: Mutual exclusion, hold and wait, no preemption, and circular wait.
5. Why is the Optimal page-replacement algorithm mainly theoretical?
   Answer: It requires knowledge of future memory references, which a real operating system does not have.