[Openmp-dev] Performance slowdown

Wed Aug 19 10:26:30 PDT 2015

Thanks Jonathan, using -fopenmp=libomp I was able to link against libomp.
However, for some benchmarks I get syntax error:

src/sparselu-task-dep.c:35:89: error: expected ']'
                        #pragma omp task firstprivate(kk) shared(BENCH)
depend(inout: BENCH[kk*matrix_size+kk:submatrix_size*submatrix_size])

                   ^

the same benchmark compile with the [older] Github version of Clang, with
clang-3.8 -fopenmp and also with GCC.

Doesn't Clang support specifying memory ranges for the dependencies or does
it just uses another syntax?

César.

On Wed, Aug 19, 2015 at 1:37 PM, Peyton, Jonathan L <
jonathan.l.peyton at intel.com> wrote:

> Currently to use libomp, the flag is: -fopenmp=libomp
>
>
>
> -- Johnny
>
>
>
> *From:* Openmp-dev [mailto:openmp-dev-bounces at lists.llvm.org] *On Behalf
> Of *César via Openmp-dev
> *Sent:* Wednesday, August 19, 2015 11:30 AM
> *To:* andreybokhanko
> *Cc:* openmp-dev at lists.llvm.org
> *Subject:* Re: [Openmp-dev] Performance slowdown
>
>
>
> 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 It's probably outdated and can't be used for reliable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>
> 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>
>
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