6.2 Process and thread synchronization primitives

Process and thread synchronization is crucial in modern operating systems. It ensures correct behavior and data integrity when multiple processes or threads run simultaneously, sharing resources like CPU and memory.

Synchronization primitives like locks, semaphores, and condition variables help coordinate access to shared resources. They prevent race conditions and implement critical sections, allowing developers to build reliable and efficient concurrent systems.

Process and Thread Synchronization

Concurrent Systems and Synchronization Needs

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• Concurrent systems involve multiple processes or threads executing simultaneously sharing resources (CPU, memory, I/O devices)
• Synchronization ensures correct program behavior and maintains data integrity in concurrent environments
• Without proper synchronization, race conditions occur leading to unpredictable results and system instability
• Synchronization mechanisms coordinate access to shared resources preventing conflicts (file system, network sockets)
• Process and thread synchronization implements critical sections where exclusive access to shared resources occurs
• Synchronization primitives control execution order and manage dependencies between concurrent tasks
  • Enable proper sequencing of operations (producer-consumer relationship)
  • Facilitate communication between threads or processes (passing data or signals)

Importance of Synchronization in Software Design

• Ensures thread-safety in multi-threaded applications preventing data corruption
• Enables efficient resource utilization by coordinating access to limited resources
• Facilitates implementation of complex algorithms requiring ordered execution (parallel sorting)
• Supports scalability in distributed systems by managing concurrent operations across multiple nodes
• Enhances reliability and fault tolerance through coordinated error handling and recovery mechanisms
• Improves overall system performance by reducing contention and optimizing resource usage

Critical Sections, Race Conditions, and Mutual Exclusion

Understanding Critical Sections and Race Conditions

• Critical section represents a segment of code accessing shared resources executed atomically to maintain consistency
• Race conditions occur when multiple threads or processes access shared data concurrently leading to incorrect results
  • Example: Two threads simultaneously incrementing a shared counter resulting in lost updates
• Proper implementation of critical sections prevents race conditions ensuring correctness of concurrent programs
• Critical section problem involves designing protocols ensuring mutual exclusion, progress, and bounded waiting
  • Mutual exclusion: Only one process can execute in the critical section at a time
  • Progress: Processes outside the critical section cannot prevent others from entering
  • Bounded waiting: There exists a bound on the number of times other processes enter their critical sections

Mutual Exclusion and Related Concepts

• Mutual exclusion ensures only one process or thread accesses a shared resource or executes a critical section at a time
• Achieved through various synchronization primitives (locks, semaphores, monitors)
• Deadlocks occur when processes are unable to proceed due to circular resource dependencies
  • Example: Two threads each holding a lock required by the other
• Livelocks happen when processes continuously change their states in response to each other without making progress
  • Example: Two people repeatedly moving aside to let the other pass in a narrow corridor
• Priority inversion occurs when a high-priority task is indirectly preempted by a lower-priority task through resource contention
  • Example: A high-priority process waiting for a resource held by a low-priority process

Synchronization Primitives

Locks and Mutex Mechanisms

• Locks provide mutual exclusion allowing only one thread to acquire the lock and enter a critical section
• Spinlocks continuously check lock availability suitable for short-duration critical sections
  • Efficient for multicore systems with low contention
  • Can waste CPU cycles in high-contention scenarios
• Mutex (mutual exclusion) locks put waiting threads to sleep reducing CPU usage
  • More efficient for longer critical sections or high-contention environments
  • Involve context switches when blocking and unblocking threads
• Readers-writer locks allow multiple concurrent readers but exclusive access for writers
  • Improve concurrency for read-heavy workloads
  • Can lead to writer starvation if not implemented carefully

Advanced Synchronization Primitives

• Barriers enable multiple threads to wait at a specific point until all threads reach that point before proceeding
  • Useful for synchronizing phases in parallel algorithms (parallel matrix multiplication)
• Condition variables allow threads to wait for a specific condition to become true before continuing execution
  • Often used with mutex locks to implement complex synchronization patterns
  • Example: Producer-consumer queue where consumers wait for items to be available
• Semaphores serve both mutual exclusion and signaling between threads or processes
  • Binary semaphores function similarly to mutex locks
  • Counting semaphores manage access to a finite pool of resources (connection pool)
• Implementing these primitives requires consideration of fairness, performance, and potential issues (priority inversion, convoy effects)

Synchronization Mechanisms: Correctness and Performance

Correctness Analysis and Deadlock Prevention

• Correctness analysis verifies synchronization mechanisms properly enforce mutual exclusion and prevent race conditions
• Deadlock detection techniques identify potential deadlock situations in concurrent systems
  • Resource allocation graphs
  • Timeout-based detection
• Deadlock prevention strategies ensure one or more necessary conditions for deadlock are never satisfied
  • Resource ordering to prevent circular wait
  • Atomic acquisition of multiple resources
• Livelock prevention techniques ensure processes make progress despite potential conflicts
  • Randomized backoff strategies
  • Priority-based conflict resolution

Performance Implications and Optimization

• Synchronization increases latency, reduces parallelism, and creates potential contention for shared resources
• Fine-grained locking improves performance allowing more concurrency but increases complexity and deadlock risk
• Lock-free and wait-free algorithms provide alternatives to traditional locking mechanisms
  • Use atomic operations and careful algorithm design to ensure progress without explicit locks
  • Can offer better performance in high-contention scenarios or on systems with many cores
• Scalability analysis examines how synchronization mechanisms perform under increased load or on many-core systems
  • Identify bottlenecks and contention points
  • Evaluate trade-offs between synchronization granularity and overhead
• Profiling and benchmarking tools measure synchronization impact on system performance
  • Identify hot spots and lock contention
  • Guide optimization efforts for improved concurrency