I was curious in benchmark of different synchronization mechanisms: atomic, spinlock, mutex.

Without synchronization

#include <future>
#include <iostream>

volatile int value = 0;

int loop(bool inc, int limit) {
  std::cout << "Started " << inc << " " << limit << std::endl;
  for (int i = 0; i < limit; ++i) {
    if (inc) { 
      ++value;
    } else {
      --value;
    }
  }
  return 0;
}

int main() {
  auto f = std::async(std::launch::async, std::bind(loop, true, 20000000));
  loop(false, 10000000);
  f.wait();
  std::cout << value << std::endl;
}

Compiling via clang:

clang++ -std=c++11 -stdlib=libc++ -O3 -o test test.cpp && time ./test

Run:

SSttaarrtteedd  10  2100000000000000

11177087

real    0m0.070s
user    0m0.089s
sys 0m0.002s

Obviously, the increment and decrement operations aren’t atomic, and the value variable contains garbage at the end.

LOCK

#include <future>
#include <iostream>

volatile int value = 0;

int loop(bool inc, int limit) {
  std::cout << "Started " << inc << " " << limit << std::endl;
  for (int i = 0; i < limit; ++i) {
    if (inc) { 
      asm("LOCK");
      ++value;
    } else {
      asm("LOCK");
      --value;
    }
  }
  return 0;
}

int main() {
  auto f = std::async(std::launch::async, std::bind(loop, true, 20000000));
  loop(false, 10000000);
  f.wait();
  std::cout << value << std::endl;
} 

Run:

SSttaarrtteedd  10  2000000100000000

10000000

real    0m0.481s
user    0m0.779s
sys 0m0.005s

Now value has the correct value at the end, but, of course, this is a dirty non-portable hack for x86 only. For example, this code works for me only if compiled with -O3. Otherwise it crashes with “illegal instruction” because the compiler injects extra stuff between the LOCK instruction and the following increment or decrement.

Atomic

#include <future>
#include <iostream>
#include "boost/interprocess/detail/atomic.hpp"

using namespace boost::interprocess::ipcdetail;

volatile boost::uint32_t value = 0;

int loop(bool inc, int limit) {
  std::cout << "Started " << inc << " " << limit << std::endl;
  for (int i = 0; i < limit; ++i) {
    if (inc) { 
      atomic_inc32(&value);
    } else {
      atomic_dec32(&value);
    }
  }
  return 0;
}

int main() {
  auto f = std::async(std::launch::async, std::bind(loop, true, 20000000));
  loop(false, 10000000);
  f.wait();
  std::cout << atomic_read32(&value) << std::endl;
}

Run:

SSttaarrtteedd  10  2100000000000000

10000000

real    0m0.457s
user    0m0.734s
sys 0m0.004s

The result is correct, and the time is almost the same as with LOCK. Not surprisingly, atomic under cover also uses LOCK but in a portable and guaranteed way.

Spinlock

#include <future>
#include <iostream>
#include "boost/smart_ptr/detail/spinlock.hpp"

boost::detail::spinlock lock;
volatile int value = 0;

int loop(bool inc, int limit) {
  std::cout << "Started " << inc << " " << limit << std::endl;
  for (int i = 0; i < limit; ++i) {
    std::lock_guard<boost::detail::spinlock> guard(lock);
    if (inc) { 
      ++value;
    } else {
      --value;
    }
  }
  return 0;
}

int main() {
  auto f = std::async(std::launch::async, std::bind(loop, true, 20000000));
  loop(false, 10000000);
  f.wait();
  std::cout << value << std::endl;
}

Run:

SSttaarrtteedd  10  2100000000000000

10000000

real    0m0.541s
user    0m0.675s
sys 0m0.089s

A bit slower but not much.

Mutex

#include <future>
#include <iostream>

std::mutex mutex;
volatile int value = 0;

int loop(bool inc, int limit) {
  std::cout << "Started " << inc << " " << limit << std::endl;
  for (int i = 0; i < limit; ++i) {
    std::lock_guard<std::mutex> guard(mutex);
    if (inc) { 
      ++value;
    } else {
      --value;
    }
  }
  return 0;
}

int main() {
  auto f = std::async(std::launch::async, std::bind(loop, true, 20000000));
  loop(false, 10000000);
  f.wait();
  std::cout << value << std::endl;
}

Run:

SSttaarrtteedd  10  2010000000000000

10000000

real    0m25.229s
user    0m7.011s
sys 0m22.667s

Now it works much slower, really.

Benchmark

Method	Time (sec.)
No synchronization	0.070
LOCK	0.481
Atomic	0.457
Spinlock	0.541
Mutex	22.667

Of course, the result depends really on the platform and the compiler (I tested on Mac Air and clang). But for me it was quite interesting to see that spinlock, in spite of its more sophisticated implementation comparing to atomics, works not much slower.

Sadly, my clang 3.1 still doesn’t support atomic, and I had to use boost.