libomp.so dependency (need REAL fix)

[eLvErDe]

Hello,

I've read nearly most related ticket which usually ends up disabling openmp. Sounds like the worst idea ever.

Is it possible to have an official statement regarding this ? mxnet 1.0.0 was building just fine with -DUSE_OPENMP=1 without adding this unresolved library dependency.

Do you know what has been changed ? Is that really solved ? By which commit, which release ?

Thanks in advance,

Best regards, Adam.

[eLvErDe]

Here's a list of related tickets/commits ending with "disabling openmp":

85215b6
#10011
#10230

[marcoabreu]

Hello Adam,
just to clarify: Are you talking about ARM? If yes, please specify which version exactly and how you are trying to compile. There have been some changes recently and we'd like to make sure we are on the same page.
Best regards,
Marco

[eLvErDe]

Absolutely not, regular X64. Multiple variants being built with/without MKL-DNN and/or CUDA.

From what I saw, the difference between 1.0.0 and 1.1.1 is in root tree CMakeLists.txt:

In 1.0.0:

  if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/openmp/CMakeLists.txt)
    # Intel/llvm OpenMP: https://github.com/llvm-mirror/openmp

In 1.1.0:

  if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/openmp/CMakeLists.txt AND SYSTEM_ARCHITECTURE STREQUAL "x86_64" AND NOT MSVC)
    # Intel/llvm OpenMP: https://github.com/llvm-mirror/openmp

The main difference is that this openmp folder DID NOT exist in 1.0.0 sources tree, so it falls back building using system openmp:

  else()
    if(OPENMP_FOUND)
      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
      set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
      set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")

Regards, Adam.

[frankfliu]

@eLvErDe thanks for the question, @sandeep-krishnamurthy requesting this be labeled under Question

[larroy]

Also related: #8532

[lebeg]

Actually libomp.so should not be a dependency at all. In the cmake file the bundled version is build and I have reasons to believe that it's not being used at all, but the one that comes from the runtime, since building with OpenMP support is more a compiler flag than just a library link.

@cjolivier01 told that the bundled version should be faster than libgomp, but I don't know how much if it's used at all.

Generally I suggest removing the libomp dependency completely and use only the version provided by the running environment.

[thomelane]

@sandeep-krishnamurthy can we get a tag of 'build' on this please

[ThomasDelteil]

@sandeep-krishnamurthy ping
@lebeg any updated on whether openmp should or shouldn't be bundled with MXNet?

[marcoabreu]

Hi, Anton is currently on vacation and will be back next week.

[asitstands]

Any update on this issue? libomp.so in 3rdparty causes a segfault in the build.ubuntu_gpu docker environment for the CI testing. The cmake build passes all unit tests for gpu but a segfault occurs at the end. Here is the stack trace.

#0  0x00007efd8c17600e in __kmp_dephash_free_entries () from /work/build/3rdparty/openmp/runtime/src/libomp.so
#1  0x00007efd8c1761a1 in __kmp_dephash_free () from /work/build/3rdparty/openmp/runtime/src/libomp.so
#2  0x00007efd8c1394f9 in __kmp_free_implicit_task () from /work/build/3rdparty/openmp/runtime/src/libomp.so
#3  0x00007efd8c121c6f in __kmp_reap_thread(kmp_info*, int) () from /work/build/3rdparty/openmp/runtime/src/libomp.so
#4  0x00007efd8c129915 in __kmp_internal_end_library () from /work/build/3rdparty/openmp/runtime/src/libomp.so
#5  0x00007efd93eb3de7 in _dl_fini () at dl-fini.c:235
#6  0x00007efd938f5ff8 in __run_exit_handlers (status=0, listp=0x7efd93c805f8 <__exit_funcs>, 
    run_list_atexit=run_list_atexit@entry=true) at exit.c:82
#7  0x00007efd938f6045 in __GI_exit (status=<optimized out>) at exit.c:104
#8  0x000000000060db0f in Py_Exit ()
#9  0x000000000060dbfa in ?? ()
#10 0x000000000060dc66 in PyErr_PrintEx ()
#11 0x000000000060ef29 in PyRun_SimpleFileExFlags ()
#12 0x000000000063fb26 in Py_Main ()
#13 0x00000000004cfeb1 in main ()

If I remove the dependency on libomp.so and just use -fopenmp, the segfault does not occur.

[lebeg]

I created a PR #12160 to remove the submodule from the build.