[Bug libgomp/78289] New: OpenMP produces high CPU load if used in more than one process

["s dot schueller at ids-imaging dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78289

            Bug ID: 78289
           Summary: OpenMP produces high CPU load if used in more than one
                    process
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s.schueller@ids-imaging.de
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

Created attachment 40014
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40014&action=edit
gcc -O2 omp.c -lpthread -fopenmp -o omp

We discovered that our application performs very bad on Linux when two or more
processes are started which use OpenMP.
I've removed line by line from the source to find the bottleneck until there
were no more lines to remove.

I attached a simple program that exposes the problem.
In the first step eight Threads are created which run a empty openmp for loop
10000 times. After that eigth Processes are created using fork() each running
the same loop 10000.

The Thread based version takes about 1 second and the fork() version 3 minutes
30 seconds on a i7 4790

Problem could be reproduced on ubuntu 12.04, 16.04

compile attachment:
gcc -O2 omp.c -lpthread -fopenmp -o omp