Multithreading Benefits, Synchronization, and Deadlock Prevention
Multithreading Advantages
The USO (User Space Option/User System Option – define the acronym if known) provides several multithreaded benefits. Programs using multiple threads are faster than those implemented with multiple processes because thread creation, context switching, and disposal generate less overhead.
- Threads share the same address space, enabling quick and efficient communication.
- Threads within the same process can easily share resources like file descriptors, timers, and signals.
In client-server environments, multithreading improves server application performance and simplifies communication between server threads.
Thread Management
Applications utilize thread facilities through a set of routines (thread packages).
Thread Implementation Modes
- User Mode Threads (UMT): Not implemented by the application or the system kernel.
- Kernel Mode Threads (KMT): Directly implemented by the system kernel.
Shared Address Space Considerations
Threads within the same process share the same address space, lacking memory access protection. This allows a thread to easily modify data used by other threads. Therefore, applications must implement communication and synchronization mechanisms to ensure safe access to shared data. However, space sharing is extremely simple and fast.
Process vs. Thread
In single-threaded environments, the process is both the resource allocation and scheduling unit. Multithreading separates these concepts.
- In a multithreaded environment, the process is the resource allocation unit (address space, file descriptors, I/O devices).
- Each thread is an independent scheduling unit. The system selects a process to execute, then one of its threads.
Concurrency Issues
Deadlock
Deadlock occurs when a process waits for a resource that will never be available or an event that will never occur. Four conditions must be met simultaneously:
- Mutual exclusion: A resource can only be allocated to one process at a time.
- Hold and wait: A process can hold allocated resources while waiting for others.
- No preemption: A resource cannot be forcibly taken from a process.
- Circular wait: A circular dependency exists where processes wait for each other’s resources.
Deadlock prevention involves ensuring at least one of these conditions is never met. However, this is often impractical. The Banker’s algorithm, while a solution, has limitations (fixed number of processes and resources), making it difficult to implement in practice.
Starvation
Starvation occurs when a process never gets access to its critical region and shared resources. Solutions involve operating system mechanisms to guarantee resource access for all requesting processes.
Synchronization
Synchronization prevents multiple processes from accessing the same resource simultaneously. While one process accesses a resource, others must wait.
Scheduling
I/O-Bound Process Favoring
I/O-bound processes are often favored in scheduling. They are less likely to be preempted by time limits and tend to maintain higher priority, while CPU-bound processes move to lower priority queues.
Adaptive Scheduling
Adaptive scheduling allows the operating system to dynamically adjust scheduling policies based on process behavior during execution.