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c++multithreadingperformancec++11stdthread

2 threads slower than 1?

I was playing around with std::thread and something weird popped up:

#include <thread>

int k = 0;

int main() {
    std::thread t1([]() { while (k < 1000000000) { k = k + 1; }});
    std::thread t2([]() { while (k < 1000000000) { k = k + 1; }});

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

    return 0;
}

When compiling the above code with no optimizations using clang++, I got the following benchmarks:

real 0m2.377s  
user 0m4.688s  
sys  0m0.005s

I then changed my code to the following: (Now using only 1 thread)

#include <thread>

int k = 0;

int main() {
    std::thread t1([]() { while (k < 1000000000) { k = k + 1; }});

    t1.join();

    return 0;
}

And these were the new benchmarks:

real 0m2.304s
user 0m2.298s
sys  0m0.003s

Why is the code utilizing 2 threads slower than the code utilizing 1?

Solution

  • You have two threads fighting over the same variable, k. So you are spending time where the processors say "Processor 1: Hey, do you know what value k has? Processor 2: Sure, here you go!", ping-ponging back and forth every few updates. Since k isn't atomic, there's also no guarantee that thread2 doesn't write an "old" value of k so that next time thread 1 reads the value, it jumps back 1, 2, 10 or 100 steps, and has to do it over again - in theory that could lead to neither of the loops every finishing, but that would require quite a bit of bad luck.