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The meaning of conditional locking: use life examples to understand the "code system" between threads in seconds

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The meaning of conditional locking: use life examples to understand the "code system" between threads in seconds

Introduction:

When learning mutex and conditional locks in C language under Linux, my brain logic entered the dead loop of "what does conditional lock lock?", so let deepseek give me three vivid examples, which I feel very helpful. I recorded here. This answer is only used to understand the meaning of conditional locks. The code accuracy needs to be verified:

text:

In a word, the core of the sentence: conditional locks (condition variables) are like "codes" between threads, allowing threads to automatically sleep or wake up under specific conditions, avoiding meaningless loop checks, and greatly saving CPU resources. The following is 5 life scenes to give you a thorough understanding!

1. Express cabinet scenario: pick up the package only after it arrives (producer-consumer model)

█ Scene Restore

  • Courier (producer thread): put parcels into the express cabinet from time to time every day
  • User (consumer thread): Want to pick up the package, but don't want to go downstairs to check every 5 minutes

█ The problem of not using conditional lock
Every 5 minutes, the user goes to the express cabinet to enter the pickup code, and finds that there is no package and goes home. The CPU is idling wildly (hustle and wait), which is both consuming (CPU resources) and inefficient.

■ Optimization using conditional locks

// Global condition variable (express cabinet notification system)
 pthread_cond_t has_package = PTHREAD_COND_INITIALIZER;
 
 // User pickup thread
 void* user_thread(void* arg) {{
     pthread_mutex_lock(&lock);
     while(the express cabinet is empty) {{
         // Enter dormant until you receive notification from the courier
         pthread_cond_wait(&has_package, &lock);
     }}
     Take out the package ();
     pthread_mutex_unlock(&lock);
 }}
 
 // Courier thread
 void* courier_thread(void* arg) {{
     pthread_mutex_lock(&lock);
     Put in the package();
     // Delivery is completed and the notification is triggered (similar to sending text messages from express cabinets)
     pthread_cond_signal(&has_package);
     pthread_mutex_unlock(&lock);
 }}

Effect: The user goes home to sleep and goes downstairs to pick up the pieces after receiving a text message - the CPU utilization rate drops from 99% to 5%.

2. Restaurant waiting scene: Only when there is an empty table can you eat

█ Scene Restore

  • Customer (Thread A): I found a full seat at the restaurant
  • Waiter (Thread B): You need to wait until the customer finishes eating before you can arrange a new seat

█ The embarrassment of traditional mutex locks
The customer asks the waiter every 1 minute: "Is there an empty seat?" The waiter keeps answering: "Not yet!" —— Both parties are tired.

█ Elegant solution for conditional locks

pthread_cond_t table_available = PTHREAD_COND_INITIALIZER;
 
 // Customer thread
 void* customer(void* arg) {{
     pthread_mutex_lock(&restaurant_lock);
     while(number of dining tables == 0) {{
         // Wait quietly for the waiter to call the number
         pthread_cond_wait(&table_available, &restaurant_lock);
     }}
     Seated for a meal ();
     pthread_mutex_unlock(&restaurant_lock);
 }}
 
 // Waiter thread
 void* waiter(void* arg) {{
     pthread_mutex_lock(&restaurant_lock);
     Clean the dining table ();
     Number of empty tables++;
     // Broadcast notifications to all waiting customers (similar to refresh the number screen)
     pthread_cond_broadcast(&table_available);
     pthread_mutex_unlock(&restaurant_lock);
 }}

Advantages: Customers drink tea and rest, and the waiter only notifies them when there is an empty table, which will increase communication efficiency by 10 times.

3. Assembly line scenario: assembly must be in order

█ Scene Restore

  • Process A (Thread 1): Production of mobile phone case
  • Process B (Thread 2): Install the screen, but the shell must be completed before starting work

█ Uncoordinated disaster
Workers in process B continue to go to the workshop A to check progress - a lot of time is wasted on the run (thread switching overhead).

█ Industrial-grade solution for conditional locks

pthread_cond_t case_ready = PTHREAD_COND_INITIALIZER;
 
 // Process B thread
 void* install_screen(void* arg) {{
     pthread_mutex_lock(&factory_lock);
     while(shell completion number == 0) {{
         // Production line workers go to the lounge to wait
         pthread_cond_wait(&case_ready, &factory_lock);
     }}
     Installation screen();
     pthread_mutex_unlock(&factory_lock);
 }}
 
 // Process A thread
 void* make_case(void* arg) {{
     pthread_mutex_lock(&factory_lock);
     Production shell();
     shell completion number ++;
     // When the bell rings, all installers will be notified
     pthread_cond_signal(&case_ready);
     pthread_mutex_unlock(&factory_lock);
 }}

Results: Workers in Process B only start work when the bell rings, and the overall production efficiency is increased by 40%.

4. Why not use mutex + loop check?

Comparative experiment: CPU occupancy test

plan CPU occupancy rate Response delay
Pure mutex + usleep(1) 25% 1ms
Conditional variables 0.3% 0.1ms 
// Error demonstration: busy waiting to consume CPU
 while(the condition is not met) {{
     pthread_mutex_unlock(&lock);
     usleep(1000); // Give up the CPU but still switch frequently
     pthread_mutex_lock(&lock);
 }}

Conclusion: Conditional variables are hibernated by kernel-level, which reduces the number of thread wakeups by 99% compared to manual sleep.

5. Summary of the core value of condition lock

  1. Eliminate busy waiting: Let the thread take the initiative to give up the CPU when the conditions are not met, instead of "occupying the pit and not shit"
  2. Accurate awakening: throughpthread_cond_signalorpthread_cond_broadcastControl wake-up strategy
  3. Avoid race conditions:pthread_cond_waitThe lock will be automatically released + enter the waiting queue to ensure that the lock can be regained after wakeup
  4. Cross-thread collaboration: Coordinate the execution order of multiple threads like a traffic light

See next timepthread_cond_wait, just imagine a thread saying, "I can't do this, so sleep for a while, call me if there is a situation!" ——This is the wisdom of an efficient programmer.

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