C++11 Thread waiting behaviour: std::this_thread::yield() vs. std::this_thread::sleep_for( std::chrono::milliseconds(1) )

Question

I was told when writing Microsoft specific C++ code that writing Sleep(1) is much better than Sleep(0) for spinlocking, due to the fact that Sleep(0)

Answer 1

I just did a test with Visual Studio 2013 on Windows 7, 2.8GHz Intel i7, default release mode optimizations.

sleep_for(nonzero) appears sleep for a minimium of around one millisecond and takes no CPU resources in a loop like:

for (int k = 0; k < 1000; ++k)
    std::this_thread::sleep_for(std::chrono::nanoseconds(1));

This loop of 1,000 sleeps takes about 1 second if you use 1 nanosecond, 1 microsecond, or 1 millisecond. On the other hand, yield() takes about 0.25 microseconds each but will spin the CPU to 100% for the thread:

for (int k = 0; k < 4,000,000; ++k) (commas added for clarity)
    std::this_thread::yield();

std::this_thread::sleep_for((std::chrono::nanoseconds(0)) seems to be about the the same as yield() (test not shown here).

In comparison, locking an atomic_flag for a spinlock takes about 5 nanoseconds. This loop is 1 second:

std::atomic_flag f = ATOMIC_FLAG_INIT;
for (int k = 0; k < 200,000,000; ++k)
    f.test_and_set();

Also, a mutex takes about 50 nanoseconds, 1 second for this loop:

for (int k = 0; k < 20,000,000; ++k)
    std::lock_guard lock(g_mutex);

Based on this, I probably wouldn't hesitate to put a yield in the spinlock, but I would almost certainly wouldn't use sleep_for. If you think your locks will be spinning a lot and are worried about cpu consumption, I would switch to std::mutex if that's practical in your application. Hopefully, the days of really bad performance on std::mutex in Windows are behind us.