No speed-up with useless printf's using OpenMP

Question

I just wrote my first OpenMP program that parallelizes a simple for loop. I ran the code on my dual core machine and saw some speed up when going from 1 thread to 2 threads. However, I ran the same code on a school linux server and saw no speed-up. After trying different things, I finally realized that removing some useless printf statements caused the code to have significant speed-up. Below is the main part of the code that I parallelized:

#pragma omp parallel for private(i)
for(i = 2; i <= n; i++)
{
  printf("useless statement");
  prime[i-2] = is_prime(i);
}

I guess that the implementation of printf has significant overhead that OpenMP must be duplicating with each thread. What causes this overhead and why can OpenMP not overcome it?

Answer 1

Speculating, but maybe the stdout is guarded by a lock?

In general, printf is an expensive operation because it interacts with other resources (such as files, the console and such).

My empirical experience is that printf is very slow on a Windows console, comparably much faster on Linux console but fastest still if redirected to a file or /dev/null.

I've found that printf-debugging can seriously impact the performance of my apps, and I use it sparingly.

Try running your application redirected to a file or to /dev/null to see if this has any appreciable impact; this will help narrow down where the problem lays.

Of course, if the printfs are useless, why are they in the loop at all?

Answer 2

To expand a bit on @Will's answer ...

I don't know whether stdout is guarded by a lock, but I'm pretty sure that writing to it is serialised at some point in the software stack. With the printf statements included OP is probably timing the execution of a lot of serial writes to stdout, not the parallelised execution of the loop.

I suggest OP modifies the printf statement to include i, see what happens.

As for the apparent speed-up on the dual-core machine -- was it statistically significant ?