[Bug libgomp/43706] New: scheduling two threads on one core leads to starvation

["baeuml at kit dot edu" <gcc-bugzilla@gcc.gnu.org>]

The following code results in starvation if at least two OpenMP threads are
assigned to one core.

#include <cstdio>

int main()
{
    while (true) {
        #pragma omp parallel for
        for (int ii = 0; ii < 1000; ++ii) {
            int s = ii;
        }
        printf(".");
    }

    return 0;
}

Compiled with
> g++ -fopenmp -save-temps -o test test.cpp

and ran with
> GOMP_CPU_AFFINITY="0 1 2" ./test

on a quad-core (Q9550) system results in veeery slow progress.


However,
> GOMP_CPU_AFFINITY="0 1 2 3" ./test
runs as expected.

This happens also, if cpu affinity is not explicitly given, but some of the
cores are busy with other processes. In this case it also helps to explicitly
assign each thread to one core with GOMP_CPU_AFFINITY="0 1 2 3".

> gcc -v
Using built-in specs.
Target: x86_64-suse-linux
Configured with: ../configure --prefix=/usr --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64
--enable-languages=c,c++,objc,fortran,obj-c++,java,ada
--enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.4
--enable-ssp --disable-libssp --with-bugurl=http://bugs.opensuse.org/
--with-pkgversion='SUSE Linux' --disable-libgcj --disable-libmudflap
--with-slibdir=/lib64 --with-system-zlib --enable-__cxa_atexit
--enable-libstdcxx-allocator=new --disable-libstdcxx-pch
--enable-version-specific-runtime-libs --program-suffix=-4.4
--enable-linux-futex --without-system-libunwind --with-arch-32=i586
--with-tune=generic --build=x86_64-suse-linux
Thread model: posix
gcc version 4.4.1 [gcc-4_4-branch revision 150839] (SUSE Linux)


-- 
           Summary: scheduling two threads on one core leads to starvation
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: baeuml at kit dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43706