Operating Systems Quiz

Logical Control Flows

• A logical control flow is a series of program counter values that relate exclusively to instructions in an executable object file or shared objects.
• The system gives each program the impression that its logical control flow runs on a dedicated computer.

Concurrency

• Concurrency occurs when multiple logical control flows are present in the system simultaneously.
• Concurrent flows overlap in execution time.
• Concurrent processes can exist even with only one processor using multitasking.
• In multitasking, time slices are allocated to multiple logical control flows, each running for a brief duration before being interrupted.
• A context switch is used to change flow from one to another.

Processes

• A process is a fundamental logical control flow abstraction.
• A process encapsulates a set of instructions, the memory those instructions use, and allocated system resources.
• Interactions with other processes occur exclusively through the operating system (OS).
• This is due to the illusion of a dedicated computer for each process.

Threads

• Threads provide a similar abstraction to processes but share more with one another.
• A process can have multiple threads.
• Threads within a single process share a memory map.
• Threads are less isolated than processes.
• Switching between threads is less expensive than switching between processes.

Multitasking and Multiprocessing

• Multitasking: Concurrent flows do not execute simultaneously, but appear to do so from each flow's viewpoint. A context switch occurs between processes interrupting one and resuming another.
• Multiprocessing:Concurrent flows can execute simultaneously, due to the presence of multiple processors in the system. Multitasking is still often used in multiprocessing situations

Concurrency and Separation

• Concurrent flows can be related or unrelated in design, implementation, memory space, resource requirements, and timing.
• When unrelated, the concept of a dedicated computer for each flow is sufficient.
• When related, the situation becomes more complicated.

Motivation for Concurrency

• Concurrency is beneficial for:
• Making progress on computational tasks blocked on slower devices.
• Achieving quick responses to specific events (like user input).
• Utilizing multiple processors.
• Simplifying design and implementation through the illusion of a dedicated computer.

Processes

• Multiple processes can proceed independently on unrelated tasks.
• They also operate independently on related tasks.
• They also are capable of cooperation on tasks.

Designing for Multiple Processes

• Multi-process designs can be reliable, as the system's isolation safeguards from errors in processes unrelated in memory.
• However, inter-process communication can be complex and expensive.
• IPC requires interacting with the operating system, which is often slow due to this extra step.

Threads vs. Processes (Comparison)

• Processes have separate kernel stacks, heaps, and data spaces.
• Threads share a single kernel stack, heap, and data space within a single process.

Threading Advantages

• Threads are typically less expensive than processes.
• They share memory maps, permissions, and operating system resources efficiently with a single process.
• Context switching between threads in the same process is more efficient than between processes.
• Inter-thread communication is simpler due to shared memory.

Threading Disadvantages

• Concurrent access to shared resources can be tricky.
• Many standard application programming interfaces (APIs) are not thread-safe.
• Breaking down the dedicated computer model for threads can make reasoning about process behavior complex.

Threading Use Cases

• Threads operate best for operations involving:
• Rapid switching between parallel tasks.
• Lots of large shared data.
• Blocking tasks where interruptions don't significantly hinder progress.
• Tasks requiring greater computational speed than a single processor can achieve.
• Multiple processes might potentially resolve some of these problems.
• The cost-benefits of using threads must be considered individually.

Summary

• Logical control flows organize execution steps within a program.
• Concurrency refers to managing multiple logical control flows concurrently.
• Multiprocessing allows simultaneous execution on multiple processor cores, with multitasking still commonly used.
• Processes and threads are distinct forms of managing concurrency.

Next Time

• Races.
• Synchronization.

References

• Operating systems book with the given URL is a required reading.
• Specific sections from another textbook are optional readings.