In the attached C sample I try to parallelize formatted writes to file by doing the formatting into a string buffer inside OpenMP parallel for and then writing the buffer to file with fwrite (inside an OpenMP ordered section). I see nice speedups with gcc, however slowdown instead with icc.
The attached code first writes the data in a serial fashion for reference, than does it in parallel and compares timings.
icc -O2 -qopenmp parawrite.c
$ OMP_NUM_THREADS=6 ./a.out
Time elapsed serial: 6.723 s
Time elapsed parallel: 9.325 s
$icc -v
icc version 19.0.1.144 (gcc version 8.2.0 compatibility)
$ gcc -O2 -fopenmp parawrite.c
$ OMP_NUM_THREADS=6 ./a.out
Time elapsed serial: 6.721 s
Time elapsed parallel: 1.206 s
$gcc --version
gcc (GCC) 8.2.0
Any ideas what is going on? BTW I see a similar (bad) behavior also with equivalent code in Fortran and ifort...
Best regards
Pavel Ondračka
