Openmp - yes or no?

Message

darkphader · Post by **darkphader** » Sat Jan 08, 2011 5:06 pm

I notice that the openmp USE flag is set in the default amd64 profile so that GCC gets built with openmp support. However it appears that to actually use openmp one needs the following make.conf:

Code: Select all

CFLAGS="${CFLAGS} -fopenmp"
LDFLAGS="${LDFLAGS} -fopenmp"

However this leads to a rebuild of very many packages (when you start getting GOMP errors you know the dependency wasn't built and linked with openmp). And it seems not all of them like to build as such.

When is openmp effective? Does it help a single processor dual core system? Has anyone built the whole system (desktop or server) with openmp support?

Thanks.

Chris

tony-curtis · Post by **tony-curtis** » Sat Jan 08, 2011 6:44 pm

OpenMP directives/pragmas are not processed by gcc unless the -fopenmp option is given, as you noted (supported != always in play). You can use it on a system with any number of cores, but you'll only see potential speedups on multi-core systems: so, yes, a dual-core system can scale to 2 threads (woo!).

How many Gentoo packages/ebuilds actually contain source code with OpenMP directives? Very few is my guess.