`std::condition_variable::wait_for` calls the predicate very often

Question

Consider the following codesnippet:

#include <iostream>
#include <condition_variable>
#include <chrono>
#include <mutex>

int main () {
  std::mutex y;
  std::condition_variable x;
  std::unique_lock<std::mutex>lock{y};
  int i = 0;
  auto increment = [&] {++i; return false;};
  using namespace std::chrono_literals;

  //lock 5s if increment returns false
  //let's see how often was increment called?
  x.wait_for(lock, 5s, increment);
  std::cout << i << std::endl;

  //compare this with a simple loop:
  //how often can my system call increment in 5s?
  auto const end = std::chrono::system_clock::now() + 5s;
  i = 0;
  while (std::chrono::system_clock::now() < end) {
    increment();
  }
  std::cout << i;
}

As I understand wait_for, i should be O(1) after wait_for (let's assume spurious unlocks are rare).

However, I get
i ~= 3e8 for kernel 4.17.14, Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz,
i ~= 8e6 for kernel 3.10.0, Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz.

This sounds funny, so i check by comparing with a simple loop that runs 5 seconds. Roughly same results for i, only a 5-10% difference.

Question:
what is wait_for doing? Does it work as expected and I just understood cppreference wrong, or did I mess up?

Second, (optional) question: Where does this enormous difference in i come from?

Additional info: (gcc7.3, gcc8.2, clang6.0), flags: -O3 --std=c++17 all yield comparable results.

Answer 1

libstdc++ has an unfortunate ability to compile and seemingly work without pthread however it wont function correctly.

See this libstdc++ bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58929

You need to add "-pthread" to your compile command.

Answer 2

You need to add -pthread flag to gcc when compiling, for example on gcc 5.1.0 on RE:

without pthread: 49752692
with pthread: 2