[Openmp-dev] Performance slowdown

[César via Openmp-dev]

On Wed, Aug 19, 2015 at 12:59 PM, andreybokhanko <andreybokhanko at gmail.com>
wrote:

> Indeed, I meant official released 3.5. Did you get your compiler from
> clang-omp.github? It's probably outdated and can't be used for reliable
> performance measurements.
>
> I will update clang-omp.github home page to avoid further confusion.
>
>
Yes, that was exactly what happend! Thanks Alexey/Andrey.

I have just downloaded clang-3.8 (trunk) and started some experiments,
however I see that clang is trying to link with "lib gomp" (GNU) and not
"libomp" (Intel). Is this really the intended default behavior? How can I
tell clang to use the Intel OMP?





> Yours,
> Andrey
>
> 19 авг. 2015 г., в 8:17, Bataev, Alexey <a.bataev at hotmail.com> написал(а):
>
> If you're using 3.5, then this is unofficial version with OpenMP. Use 3.7
> instead just like Andrey said
>
> Best regards,
> Alexey Bataev
> =============
> Software Engineer
> Intel Compiler Team
>
> 19.08.2015 1:25, César via Openmp-dev пишет:
>
> Hi Andrey,
>
> this is strange because when I compile with "clang-3.5 -fopenmp" the
> executable that is produced is parallel. I am sure of this because I'm able
> to see the threads and also because I can see the symbols used by the IOMP
> runtime in the binary.
>
> $ clang -O3 -g -fopenmp toy13.cpp -o toy13 -lm
>
> $ nm toy13 | grep kmpc
> U __kmpc_cancel_barrier@@VERSION
> U __kmpc_end_single@@VERSION
> U __kmpc_fork_call@@VERSION
> U __kmpc_omp_task_alloc@@VERSION
> U __kmpc_omp_task_with_deps@@VERSION
> U __kmpc_single@@VERSION
>
> $ ldd toy13
> linux-vdso.so.1 =>  (0x00007fff9805d000)
> libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fc00e3cc000)
> libiomp5.so => /usr/lib/libiomp5.so (0x00007fc00e121000)
> libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0
> (0x00007fc00df03000)
> libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fc00db3e000)
> libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fc00d939000)
> /lib64/ld-linux-x86-64.so.2 (0x00007fc00e6fc000)
>
>
>
>
>
> César.
>
> On Tue, Aug 18, 2015 at 6:15 PM, <andreybokhanko at gmail.com> wrote:
>
>> César,
>>
>> - The version of Clang that I am using to compile the benchmarks: 3.5.0
>>
>>
>> Clang 3.5 doesn't support OpenMP -- it simply ignores the pragmas.
>>
>> Please use version from trunk or from 3_7 release branch. Also, please
>> supply -fopenmp= libomp option.
>>
>> Yours,
>> Andrey Bokhanko
>> =============
>> Software Engineer
>> Intel Compiler Team
>> Intel
>>
>> Отправлено с iPad
>>
>> 18 авг. 2015 г., в 21:14, César via Openmp-dev <
>> <openmp-dev at lists.llvm.org>openmp-dev at lists.llvm.org> написал(а):
>>
>> 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.
>>
>> <pic1.png>
>>
>> <pic2.png>
>>
>> <pic3.png>
>>
>> _______________________________________________
>> Openmp-dev mailing list
>> Openmp-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev
>>
>>
>
>
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