No speed-up with useless printf's using OpenMP
multithreading, openmp, parallel-processing, performance, printf
Solution
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?
Problem
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?