Performance Comparison of Condition Variables and Atomics in C++20

11 January 2021

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After the introduction to std::atomic_flag in my last post, Synchronization with Atomics in C++20, I want to dive deeper. Today, I created a ping-pong game using condition variables, std::atomic_flag and std::atomic<bool>. Let's play.

TimelineCpp20CoreLanguage

The key question I want to answer in this post is: What is the fastest way to synchronize threads in C++20? I use in this post three different data types: std::condition_variable, std::atomic_flag, and std::atomic<bool>.

To get comparable numbers, I implement a ping-pong game. One thread executes a ping function, and the other thread a pong function. For simplicity reasons, I call the thread executing the ping function the ping thread and the other thread the pong thread. The ping thread waits for the notification of the pong threads and returns the notification to the pong thread. The game stops after 1,000,000 ball changes. I perform each game five times to get comparable performance numbers.

I made my performance test with the brand new Visual Studio compiler because it already supports synchronization with atomics. Additionally, I compiled the examples with maximum optimization (/Ox).

windowsCompiler

Let me start with C++11.

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Condition Variables

// pingPongConditionVariable.cpp

#include <condition_variable>
#include <iostream>
#include <atomic>
#include <thread>

bool dataReady{false};

std::mutex mutex_;
std::condition_variable condVar1;          // (1)
std::condition_variable condVar2;          // (2)

std::atomic<int> counter{};
constexpr int countlimit = 1'000'000;

void ping() {

    while(counter <= countlimit) {
        {
            std::unique_lock<std::mutex> lck(mutex_);
            condVar1.wait(lck, []{return dataReady == false;});
            dataReady = true;
        }
        ++counter;                          
        condVar2.notify_one();              // (3)
  }
}

void pong() {

    while(counter < countlimit) {  
        {
            std::unique_lock<std::mutex> lck(mutex_);
            condVar2.wait(lck, []{return dataReady == true;});
            dataReady = false;
        }
        condVar1.notify_one();            // (3)
  }

}

int main(){

    auto start = std::chrono::system_clock::now();  

    std::thread t1(ping);
    std::thread t2(pong);

    t1.join();
    t2.join();
  
    std::chrono::duration<double> dur = std::chrono::system_clock::now() - start;
    std::cout << "Duration: " << dur.count() << " seconds" << std::endl;

}

I use two condition variables in the program: condVar1 and condVar2 (lines 1 and 2). The ping thread waits for the notification of condVar1 and sends its notification with condVar2. dataReady protects against spurious and lost wakeups (see "C++ Core Guidelines: Be Aware of the Traps of Condition Variables"). The ping-pong game ends when counter reaches the countlimit. The nofication_one calls (lines 3) and the counter are thread-safe and are, therefore, outside the critical region.

Here are the numbers:

pingPongConditionVariable

The average execution time is 0.52 seconds.

Porting this play to std::atomic_flags's in C++20 is straightforward.

`std::atomic_flag`

Here is the play using two atomic flags.

Two Atomic Flags

In the following program, I replace the waiting on the condition variable with the waiting on the atomic flag and the notification of the condition variable with the setting of the atomic flag followed by the notification.

// pingPongAtomicFlags.cpp

#include <iostream>
#include <atomic>
#include <thread>

std::atomic_flag condAtomicFlag1{};
std::atomic_flag condAtomicFlag2{};

std::atomic<int> counter{};
constexpr int countlimit = 1'000'000;

void ping() {
    while(counter <= countlimit) {
        condAtomicFlag1.wait(false);               // (1)
        condAtomicFlag1.clear();                   // (2)

        ++counter;
        
        condAtomicFlag2.test_and_set();           // (4)
        condAtomicFlag2.notify_one();             // (3)
    }
}

void pong() {
    while(counter < countlimit) {
        condAtomicFlag2.wait(false);
        condAtomicFlag2.clear();
        
        condAtomicFlag1.test_and_set();
        condAtomicFlag1.notify_one();
    }
}

int main() {

     auto start = std::chrono::system_clock::now();  

    condAtomicFlag1.test_and_set();                    // (5)
    std::thread t1(ping);
    std::thread t2(pong);

    t1.join();
    t2.join();

    std::chrono::duration<double> dur = std::chrono::system_clock::now() - start;
    std::cout << "Duration: " << dur.count() << " seconds" << std::endl;

}

A call condAtomicFlag1.wait(false) (1) blocks, if the value of the atomic flag is false. On the contrary, it returns if condAtomicFlag1 has the value true. The boolean value serves as a kind of predicate and must, therefore, be set back to false (2). Before the notification (3) is sent to the pong thread, condAtomicFlag1 is set to true (4). The initial setting of condAtomicFlag1 to true (5) starts the game.

Thanks to std::atomic_flag the game end earlier.

pingPongAtomicFlags

On average, a game takes 0.32 seconds.

When you analyze the program, you may recognize that one atomics flag is sufficient for the play.

One Atomic Flag

Using one atomic flag makes the play easier to understand.

// pingPongAtomicFlag.cpp

#include <iostream>
#include <atomic>
#include <thread>

std::atomic_flag condAtomicFlag{};

std::atomic<int> counter{};
constexpr int countlimit = 1'000'000;

void ping() {
    while(counter <= countlimit) {
        condAtomicFlag.wait(true);
        condAtomicFlag.test_and_set();
        
        ++counter;
        
        condAtomicFlag.notify_one();
    }
}

void pong() {
    while(counter < countlimit) {
        condAtomicFlag.wait(false);
        condAtomicFlag.clear();
        condAtomicFlag.notify_one();
    }
}

int main() {

     auto start = std::chrono::system_clock::now();  

    
    condAtomicFlag.test_and_set();
    std::thread t1(ping);
    std::thread t2(pong);

    t1.join();
    t2.join();

    std::chrono::duration<double> dur = std::chrono::system_clock::now() - start;
    std::cout << "Duration: " << dur.count() << " seconds" << std::endl;

}

In this case, the ping thread blocks on true but the pong thread blocks on false. Using one or two atomic flags make no difference from the performance perspective.

pingPongAtomicFlag

The average execution time is 0.31 seconds.

I used in this example std::atomic_flag such as an atomic boolean. Let's give it another try with std::atomic<bool>.

`std::atomic<bool>`

From the readability perspective, I prefer the following C++20 implementation based on std::atomic<bool>.

// pingPongAtomicBool.cpp

#include <iostream>
#include <atomic>
#include <thread>

std::atomic<bool> atomicBool{};

std::atomic<int> counter{};
constexpr int countlimit = 1'000'000;

void ping() {
    while(counter <= countlimit) {
        atomicBool.wait(true);
        atomicBool.store(true);

        ++counter;
        
        atomicBool.notify_one();
    }
}

void pong() {
    while(counter < countlimit) {
        atomicBool.wait(false);
        atomicBool.store(false);
        atomicBool.notify_one();
    }
}

int main() {

    std::cout << std::boolalpha << std::endl;

    std::cout << "atomicBool.is_lock_free(): "              // (1)
              << atomicBool.is_lock_free()  << std::endl; 

    std::cout << std::endl;

    auto start = std::chrono::system_clock::now();

    atomicBool.store(true);
    std::thread t1(ping);
    std::thread t2(pong);

    t1.join();
    t2.join();

    std::chrono::duration<double> dur = std::chrono::system_clock::now() - start;
    std::cout << "Duration: " << dur.count() << " seconds" << std::endl;

}

std::atomic<bool> can internally use a locking mechanism such as a mutex. As I assumed, my Windows runtime is lock-free (1).

pingPongAtomicBool

On average, the execution time is 0.38 seconds.

All Numbers

As expected, condition variables are the slowest way, and atomic flag is the fastest way to synchronize threads. The performance of a std::atomic<bool> is in-between.

PerformanceComparison

What's next?

With C++20, we have a few new mechanisms for thread coordination. In my next post, I will look deeper into latches, barriers, and semaphores. They also allow it to play Ping-Pong.

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Tags: Atomics, condition variables

Comments

0 #11 rjkuro 2021-06-21 01:16

I tried condition variables version on Linux and Windows. The performance on Linux was about 20 times slower (12 secs) than that on Windows (0.57 secs). What do you think makes Windows condition variables so fast?

Quote

+1 #12 Nate Eldredge 2022-04-17 06:56

I believe all three versions have a race condition at the end of the loop that can result in deadlock.

With counter == 999999 and ping() sleeping, suppose pong() notifies the condition variable and then stalls for a while. ping() wakes up, does its thing, and increments the counter. Now pong() wakes back up, does the test counter < countlimit, finds it false, and exits. Meanwhile ping() goes back around for another iteration, but it will sleep forever as pong() is no longer there to wake it up.

I think that both ping() and pong() ought to have the same exit condition (either < or