[Openmp-dev] Performance slowdown

[John Mellor-Crummey via Openmp-dev]

My guess is that you are blocking rather than spinning. Using OMP_WAIT_POLICY=active doesn't seem to be enough with Intel's runtime to turn off all blocking. As I recall, there is a KMP_xxx flag that applies as well. You can grep the barrier code in the runtime or hope that someone from intel responds to your inquiry.

--
John Mellor-Crummey

(sent from my phone)

> On Aug 18, 2015, at 1:14 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
> 
> 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.
> !DSPAM:8504,55d37631260061683114033!
> <pic1.png>
> <pic2.png>
> <pic3.png>
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