Question

I have a loop that I’m trying to parallelize and in it I am filling a container, say an STL map. Consider then the simple pseudo code below where T1 and T2 are some arbitrary types, while f and g are some functions of integer argument, returning T1, T2 types respectively:

#pragma omp parallel for schedule(static) private(i) shared(c)
for(i = 0; i < N; ++i) {
   c.insert(std::make_pair<T1,T2>(f(i),g(i))
}

This looks rather straighforward and seems like it should be trivially parallelized but it doesn’t speed up as I expected. On the contrary it leads to run-time errors in my code, due to unexpected values being filled in the container, likely due to race conditions. I’ve even tried putting barriers and what-not, but all to no-avail. The only thing that allows it to work is to use a critical directive as below:

#pragma omp parallel for schedule(static) private(i) shared(c)
for(i = 0; i < N; ++i) {
#pragma omp critical
   {
      c.insert(std::make_pair<T1,T2>(f(i),g(i))
   }
}

But this sort of renders useless the whole point of using omp in the above example, since only one thread at a time is executing the bulk of the loop (the container insert statement). What am I missing here? Short of changing the way the code is written, can somebody kindly explain?

Answer 1

This particular example you have is not a good candidate for parallelism unless f() and g() are extremely expensive function calls.

1. STL containers are not thread-safe. That’s why you’re getting the race conditions. So accessing them needs to be synchronized – which makes your insertion process inherently sequential.

2. As the other answer mentions, there’s a LOT of overhead for parallelism. So unless f() and g() extremely expensive, your loop doesn’t do enough work to offset the overhead of parallelism.

Now assuming f() and g() are extremely expensive calls, then your loop can be parallelized like this:

#pragma omp parallel for schedule(static) private(i) shared(c)
    for(i = 0; i < N; ++i) {
        std::pair<T1,T2> p = std::make_pair<T1,T2>(f(i),g(i));

#pragma omp critical
       {
          c.insert(p);
       }
    }