Manual unlocking done before notifying

Question

Thanks to the book of the Doctor Scott Meyers, page 263, I recently discovered condition_variable, so I had to search about it on cppreference to study it more.

https://en.cppreference.com/w/cpp/thread/condition_variable

I have several questions on it, because I think on it since several days, and I still did not understand.

My questions are about this piece of code :

// Manual unlocking is done before notifying, to avoid waking up
// the waiting thread only to block again (see notify_one for details)
lk.unlock();
cv.notify_one();

1) I do not understand what the author of the cppreference means by that comment and by "the waiting thread, only to block again" because I don't know even how to translate it, and

2) Which threads it denotes exactly, and why in particular.

3) Does it denote the thread_worker or the main thread (parent) ?

4) What they chose to do that ?

And what does it change if the authors notify first and then manually unlock ?

Answer 1

That's a minor and usually irrelevant optimization. The concern arises because each thread that wakes up after a call to notify or notify_all has to lock the mutex before it is allowed to proceed. If the call to unlock occurs after the call to notify_one (or to notify), the thread(s) that wake up will have to wait until after the calling thread unlocks it. If the call to unlock is made before the notify call then one awakened thread can get the mutex immediately.