Operating Systems Lecture 14: Condition variables

Condition Variables in Thread Synchronization

Introduction to Condition Variables

• The lecture introduces condition variables as a synchronization primitive for threads, complementing locks used for mutual exclusion.

• It highlights the need for waiting and signaling between threads, where one thread may need another to complete a task before proceeding.

Inefficiency of Busy Waiting

• Busy waiting is described as an inefficient method where a thread continuously checks a variable until it changes, leading to wasted CPU cycles.

• Condition variables are presented as a more efficient alternative that allows threads to wait and be signaled when conditions change.

Understanding Condition Variables

• A condition variable acts like a queue where threads can place themselves in a waiting state until they are signaled by another thread.

• The operating system also utilizes similar mechanisms for kernel threads, allowing processes to wait for events or signals from other processes.

Signaling Mechanisms

• There are two types of signaling: simple signal (wakes one waiting thread) and broadcast signal (wakes all waiting threads).

Example of Parent-Child Thread Synchronization

• An example illustrates how a parent thread waits for its child thread to finish execution using P-thread conditional variables.

• The parent creates the child thread and calls the join function, which checks if the child has completed; if not, it waits on the condition variable.

Execution Flow with Condition Variables

• The parent checks if the shared variable done is zero (indicating the child hasn't finished), then goes into sleep mode by calling wait on the condition variable.

• Once the child completes its task, it sets done to one and signals the condition variable, waking up the parent thread.

Ensuring Correct Execution Order

• Regardless of scheduling order by the operating system, using condition variables ensures that the parent will always execute after the child has completed its work.

Importance of Checking Conditions Before Waiting

• It's crucial for parents to check conditions before calling wait; if done is already one, there's no need to sleep since they won't be woken up again.

Best Practices with Condition Variables

• Always check conditions before entering wait states. If you enter wait without checking and another signal has already occurred, you risk missing wake-up calls.

Conclusion on Conditional Variable Usage

Understanding Condition Variables in Programming

Importance of Using While Loops with Condition Variables

• It is crucial to use a while loop for checking conditions before waiting, as some library implementations may cause spurious wake-ups, waking the thread even if the condition remains false.

• The ideal behavior after returning from a wait is that the condition should be true; however, using a while loop ensures that the condition is re-evaluated upon waking up.

• This practice of checking conditions within a while loop is considered good programming practice and standard structure when working with condition variables.

Role of Locks in Thread Synchronization

• A mutex lock must be held during the check and wait process to prevent race conditions, even if there are no shared variable updates occurring.

• If a parent thread checks a condition and decides to sleep but gets interrupted before sleeping, it can lead to scenarios where the child thread signals without anyone being awake to receive it.

• This interruption can result in the parent sleeping indefinitely because it was not properly synchronized with the child's signaling action.

Atomicity in Sleep/Wake Processes

• To avoid issues where threads might miss signals due to interruptions, both deciding to sleep and actually going to sleep must occur atomically.

• Holding a lock before checking conditions ensures that once a decision is made to sleep, no other thread can interfere until after control has been transferred away from the parent thread.

Producer-Consumer Problem Overview

• The producer-consumer problem illustrates how multiple threads (producers and consumers) interact through a shared buffer, necessitating mutual exclusion for safe access.

• Signaling mechanisms are essential; producers should wait when buffers are full and consumers should wait when buffers are empty. Condition variables facilitate this signaling between threads.

Application of Condition Variables

• In multi-threaded applications like web servers, one thread may read requests into a queue (producer), while another processes these requests (consumer).

• The implementation requires careful management of locks around waiting/sleeping states to ensure smooth operation without deadlocks or missed signals.

Producer-Consumer Problem and Condition Variables

Understanding the Producer-Consumer Model

• The producer-consumer model involves a list that can reach a maximum value. When the list is full, the producer must wait on an empty condition variable until space becomes available.

• It is crucial to match wait and signal statements when using condition variables. A wait statement without a corresponding signal can lead to indefinite sleeping of the thread.

• The consumer also waits on a condition variable if the buffer is empty. Once the producer produces an item, it signals this condition variable to notify the consumer that there is something available for consumption.

Synchronization Mechanisms

• Producers wait when the buffer is full, while consumers signal when they create empty space. Conversely, consumers wait on an empty buffer and are signaled by producers once items are produced.