[Openmp-dev] Performance slowdown

[Jack Howarth via Openmp-dev]

On Tue, Aug 18, 2015 at 2:14 PM, César via Openmp-dev
<openmp-dev at lists.llvm.org> wrote:
> Hello,
>
> I don't know if this is the correct list to talk about this - I did not find
> a better place..
>
> I am doing performance experiments with a few OpenMP implementations (IOMP,
> GOMP and our private impl.) and I am seeing a severe slowdown when I use
> IOMP (GOMP and others are performing well).
>
> The benchmarks I am using are these ones:
> http://kastors.gforge.inria.fr/#!index.md

That web page claims the benchmarks use parts of the OpenMP 4.0 specification.

"The KaStORS benchmark suite has been designed to evaluate the implementation of
the OpenMP dependent task paradigm, introduced as part of the OpenMP 4.0
specification."

Currently openmp is only complete for the OpenMP 3.2 specification

>
> Really, the slowdown is huge. For one of the programs (plasma/dpotrf_taskdep
> -n 8192 -b 64 -i 1 -c) the serial version executes in ~28s and the parallel
> one executes in ~110s. I did some profiling and found that most of the time
> is being spent on synchronization barriers and dependence tracking (see
> attached image). Before digging deeper I would like to hear back from you if
> I am doing something wrong here:
>
> - I tested with the last version of the repository:
> http://llvm.org/svn/llvm-project/openmp/trunk
> - I am using Ubuntu 14.10.
> - I have tested on more than one machine, the results above are from a Intel
> i7-3770
> - The runtime itself is compiled using: make compiler=gcc os_omp=linux
> arch=32e
> - The version of GCC that I am using is: 4.9.1
> - The version of Clang that I am using to compile the benchmarks: 3.5.0
>
>
> César.
>
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