What is the difference between atomic and critical in OpenMP?

Question

What is the difference between atomic and critical in OpenMP?

I can do this

#pragma omp atomic
g_qCount++;

but isn't this same as

#pragma omp critical
g_qCount++;

?

Answer 1

The effect on g_qCount is the same, but what's done is different.

An OpenMP critical section is completely general - it can surround any arbitrary block of code. You pay for that generality, however, by incurring significant overhead every time a thread enters and exits the critical section (on top of the inherent cost of serialization).

(In addition, in OpenMP all unnamed critical sections are considered identical (if you prefer, there's only one lock for all unnamed critical sections), so that if one thread is in one [unnamed] critical section as above, no thread can enter any [unnamed] critical section. As you might guess, you can get around this by using named critical sections).

An atomic operation has much lower overhead. Where available, it takes advantage on the hardware providing (say) an atomic increment operation; in that case there's no lock/unlock needed on entering/exiting the line of code, it just does the atomic increment which the hardware tells you can't be interfered with.

The upsides are that the overhead is much lower, and one thread being in an atomic operation doesn't block any (different) atomic operations about to happen. The downside is the restricted set of operations that atomic supports.

Of course, in either case, you incur the cost of serialization.