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From Anton Chernov <mecher...@gmail.com>
Subject Re: [Discussion] Remove bundled llvm OpenMP
Date Tue, 12 Feb 2019 10:22:06 GMT
Recent benchmarking results have been published here [1]. Experiments
compare different OpenMP implementations as well as binaries compiled with
different compilers including GCC, Clang and ICC.

During experimentation another issues with mixing up libraries was
identified and described here [2].

Best
Anton

[1] https://cwiki.apache.org/confluence/x/2wclBg
[2]
https://github.com/apache/incubator-mxnet/issues/14087#issuecomment-461734041


вс, 9 дек. 2018 г. в 16:28, Anton Chernov <mechernov@gmail.com>:

> Hi Chris,
>
> Following up on the issue, are all things resolved in the discussion?
>
> If yes, I kindly ask you to reopen this PR and remove ‘requesting changes’
> status:
> https://github.com/apache/incubator-mxnet/pull/12160
>
> Thank you.
>
>
> Best
> Anton
>
>
> вт, 27 нояб. 2018 г. в 17:15, Anton Chernov <mechernov@gmail.com>:
>
>> Another thing to take into consideration:
>>
>> All python artefacts that are created (PyPi) are built with make and are
>> not using the bundled OpenMP library.
>>
>> One step for the switch to CMake to happen is the approval and merging of
>> the mentioned PR:
>>
>> https://github.com/apache/incubator-mxnet/pull/12160
>>
>> If there are no other objections I kindly ask Chris Olivier to remove his
>> 'requesting changes' veto on it to unblock the CMake overhaul work.
>>
>> Thank you.
>>
>> Best
>> Anton
>>
>> чт, 22 нояб. 2018 г. в 17:11, Anton Chernov <mechernov@gmail.com>:
>>
>>>
>>> Thank you for you answer, Chris.
>>>
>>> > The whole “mixing omp libraries” is something that occurs in production
>>> every day and certainly in everything that uses mkl.
>>>
>>> I'm afraid this statement is wrong. Intel MKL-DNN strictly ensures that
>>> this mixture is not happening:
>>>
>>> "Intel MKL-DNN uses OpenMP* for parallelism and requires an OpenMP
>>> runtime library to work. As different OpenMP runtimes may not be binary
>>> compatible it's important to ensure that only one OpenMP runtime is used
>>> throughout the application. Having more than one OpenMP runtime initialized
>>> may lead to undefined behavior resulting in incorrect results or crashes."
>>> [1]
>>>
>>> That is why 2 different MKLML libraries are provided:
>>>
>>> lib/libmklml_gnu.so  | Intel MKL small library for GNU* OpenMP runtime
>>> lib/libmklml_intel.so | Intel MKL small library for Intel(R) OpenMP
>>> runtime
>>>
>>> > is the suggestion that libiomp be removed from mkl?
>>>
>>> That is certainly not my suggestion.
>>>
>>> > have you spoken with intel? have you consulted Intel at all?
>>>
>>> Yes, I have asked for comments on the issue.
>>>
>>> > “hard to debug random crash”. you’re seeing an assertion which is
>>> probably ...
>>>
>>> I'm seeing the result of undefined behaviour. And I want to put emphasis
>>> on the following statement:
>>>
>>> I disregards of whether there is a particular reason for the assert - it
>>> is a result of behaviour that should not happen. There are valid ways how
>>> to use llvm OpenMP in MXNet and the current way is not one of them.
>>>
>>> > The lack of root-causing the problem and knee-jerk solution here makes
>>> me
>>> uncomfortable.
>>>
>>> I hope that my efforts highlighting the problems reach you to mitigate
>>> your uncomfort.
>>>
>>> > if you want to see performance differences there’s an environment
>>> variable
>>> you can set in the mxnet omp tuning code that will print overhead and
>>> execution times for the current omp library.
>>>
>>> I don't want to see performance differences in the current OpenMP
>>> library. I want to remove the current OpenMP library and use the one
>>> provided by the compiler.
>>>
>>>
>>>
>>> Best
>>> Anton
>>>
>>> [1] https://github.com/intel/mkl-dnn/blame/master/README.md#L261-L265
>>>
>>> чт, 22 нояб. 2018 г. в 16:50, Chris Olivier <cjolivier01@apache.org>:
>>>
>>>> Do you not work on CI mostly? My apologies for thinking that was some
>>>> sort
>>>> of team effort between you and a few others that were passionate about
>>>> CI
>>>> keeping the CI system running smoothly.
>>>>
>>>> You have source code, you have the line the assertion is on. If you
>>>> can’t
>>>> describe what’s going wrong that causes the assertion, then I don’t
>>>> really
>>>> have anything more to add to this conversation beyond what’s below:
>>>>
>>>> The whole “mixing omp libraries” is something that occurs in production
>>>> every day and certainly in everything that uses mkl.  It may
>>>> occasionally
>>>> cause problems for some edge cases when there is super-complex linking
>>>> strategies and dynamic loading.  But this is not one of those edge
>>>> cases.
>>>> Mostly blaming this is a red herring for other thread-related problems
>>>> and
>>>> people switch omp library and the timing of their code changes and they
>>>> stop seeing the problem. I’ve spent my entire career doing heavily
>>>> multiphreaded c++ development and i’ve seen that a million times.  is
>>>> the
>>>> suggestion that libiomp be removed from mkl? have you spoken with intel?
>>>> have you consulted Intel at all?
>>>>
>>>> and what you are seeing isn’t some “hard to debug random crash”. you’re
>>>> seeing an assertion which is probably related to omp trying to create a
>>>> thread pool after a fork and something was done in the mxnet code to
>>>> make
>>>> that sketchy to do. I’d suggest filing an issue with the llvm openmp
>>>> just
>>>> like you’d file with any other not-well-understood behavior in mxnet.
>>>>
>>>> The lack of root-causing the problem and knee-jerk solution here makes
>>>> me
>>>> uncomfortable.
>>>>
>>>> if you want to see performance differences there’s an environment
>>>> variable
>>>> you can set in the mxnet omp tuning code that will print overhead and
>>>> execution times for the current omp library.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Thu, Nov 22, 2018 at 7:12 AM Anton Chernov <mechernov@gmail.com>
>>>> wrote:
>>>>
>>>> > Hi Chris,
>>>> >
>>>> > Thank you for your answer. If you have noticed the initial email
>>>> comes from
>>>> > me, Anton Chernov (@lebeg on Github) and thus the proposal is not
>>>> from any
>>>> > 'Ci' team that you've mentioned, but from me personally.
>>>> >
>>>> > You are writing:
>>>> >
>>>> > > someone is doing something unhealthy when they fork ...
>>>> >
>>>> > I'm missing any context to understand what you mean.
>>>> >
>>>> > > we get a lot of performance gain from OMP ...
>>>> >
>>>> > There is no data that would prove this statement and therefore it is
a
>>>> > random guess.
>>>> >
>>>> > > in many months, no investigation has occurred as to WHY the
>>>> assertion is
>>>> > failing.
>>>> >
>>>> > The investigation has concluded that this is happening due to
>>>> undefined
>>>> > behaviour which is, in my opinion, a suffient answer that does not
>>>> require
>>>> > to go any deeper.
>>>> >
>>>> > > the pr is vetoed until such a time that the actual root cause of
the
>>>> > problem is known.
>>>> >
>>>> > And considering the statements above there is no valid reason to veto
>>>> the
>>>> > PR.
>>>> >
>>>> >
>>>> > Best
>>>> > Anton
>>>> >
>>>> > чт, 22 нояб. 2018 г. в 15:38, Chris Olivier <cjolivier01@gmail.com>:
>>>> >
>>>> > > 3x less overhead*
>>>> > >
>>>> > > On Thu, Nov 22, 2018 at 6:25 AM Chris Olivier <
>>>> cjolivier01@gmail.com>
>>>> > > wrote:
>>>> > >
>>>> > > > someone is doing something unhealthy when they fork, which
is
>>>> causing
>>>> > an
>>>> > > > assertion in the openmp library. the same assertion that would
>>>> fire in
>>>> > > mkl,
>>>> > > > which is linked to libiomp5 (exact same omp library). this
is new
>>>> > > behavior
>>>> > > > and most likely due to an error or suboptimal approach in
the
>>>> forking
>>>> > > logic
>>>> > > > in mxnet.
>>>> > > >
>>>> > > > in order to circumvent the assert, the Ci team is proposing
to
>>>> remove
>>>> > the
>>>> > > > library completely which is equivalent to cutting off your
leg to
>>>> make
>>>> > > the
>>>> > > > pain from stubbing your toe go away.
>>>> > > >
>>>> > > > we get a lot of performance gain from OMP. is has about a
1/3 less
>>>> > > > overhead for entering omp regions and also supports omp regions
>>>> after a
>>>> > > > fork, which libgomp does not.
>>>> > > >
>>>> > > > in many months, no investigation has occurred as to WHY the
>>>> assertion
>>>> > is
>>>> > > > failing.
>>>> > > >
>>>> > > > the pr is vetoed until such a time that the actual root cause
of
>>>> the
>>>> > > > problem is known.
>>>> > > >
>>>> > > >
>>>> > > > thanks,
>>>> > > >
>>>> > > > -Chris.
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > >
>>>> > > > On Thu, Nov 22, 2018 at 4:36 AM Anton Chernov <
>>>> mechernov@gmail.com>
>>>> > > wrote:
>>>> > > >
>>>> > > >> Dear MXNet community,
>>>> > > >>
>>>> > > >> I would like to drive attention to an important issue
that is
>>>> present
>>>> > in
>>>> > > >> the MXNet CMake build: usage of bundled llvm OpenMP library.
>>>> > > >>
>>>> > > >> I have opened a PR to remove it:
>>>> > > >> https://github.com/apache/incubator-mxnet/pull/12160
>>>> > > >>
>>>> > > >> The issue was closed, but I am strong in my oppinion that
it's
>>>> the
>>>> > right
>>>> > > >> thing to do.
>>>> > > >>
>>>> > > >> *Background*
>>>> > > >> If you want to use OpenMP pragmas in your code for
>>>> parallelization you
>>>> > > >> would supply a special flag to the compiler:
>>>> > > >>
>>>> > > >> - Clang / -fopenmp
>>>> > > >> https://openmp.llvm.org/
>>>> > > >>
>>>> > > >> - GCC / -fopenmp
>>>> > > >> https://gcc.gnu.org/onlinedocs/libgomp/Enabling-OpenMP.html
>>>> > > >>
>>>> > > >> - Intel / [Q]openmp
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://software.intel.com/en-us/node/522689#6E24682E-F411-4AE3-A04D-ECD81C7008D1
>>>> > > >>
>>>> > > >> - Visual Studio: /openmp (Enable OpenMP 2.0 Support)
>>>> > > >> https://msdn.microsoft.com/en-us/library/tt15eb9t.aspx
>>>> > > >>
>>>> > > >> Each of the compilers would enable the '#pragma omp' directive
>>>> during
>>>> > > >> C/C++
>>>> > > >> compilation and arrange for automatic linking of the OpenMP
>>>> runtime
>>>> > > >> library
>>>> > > >> supplied by each complier separately.
>>>> > > >>
>>>> > > >> Thus, to use the advantages of an OpenMP implementation
one has
>>>> to
>>>> > > compile
>>>> > > >> the code with the corresponding compiler.
>>>> > > >>
>>>> > > >> Currently, in MXNet CMake build scripts a bundled version
of llvm
>>>> > OpenMP
>>>> > > >> is
>>>> > > >> used ([1] and [2]) to replace the OpenMP library supplied
by the
>>>> > > compiler.
>>>> > > >>
>>>> > > >> I will quote here the README from the MKL-DNN (Intel(R)
Math
>>>> Kernel
>>>> > > >> Library
>>>> > > >> for Deep Neural Networks):
>>>> > > >>
>>>> > > >> "Intel MKL-DNN uses OpenMP* for parallelism and requires
an
>>>> OpenMP
>>>> > > runtime
>>>> > > >> library to work. As different OpenMP runtimes may not
be binary
>>>> > > compatible
>>>> > > >> it's important to ensure that only one OpenMP runtime
is used
>>>> > throughout
>>>> > > >> the application. Having more than one OpenMP runtime initialized
>>>> may
>>>> > > lead
>>>> > > >> to undefined behavior resulting in incorrect results or
>>>> crashes." [3]
>>>> > > >>
>>>> > > >> And:
>>>> > > >>
>>>> > > >> "Using GNU compiler with -fopenmp and -liomp5 options
will link
>>>> the
>>>> > > >> application with both Intel and GNU OpenMP runtime libraries.
>>>> This
>>>> > will
>>>> > > >> lead to undefined behavior of the application." [4]
>>>> > > >>
>>>> > > >> As can be seen from ldd for MXNet:
>>>> > > >>
>>>> > > >> $ ldd build/tests/mxnet_unit_tests | grep omp
>>>> > > >>     libomp.so =>
>>>> > /.../mxnet/build/3rdparty/openmp/runtime/src/libomp.so
>>>> > > >> (0x00007f697bc55000)
>>>> > > >>     libgomp.so.1 => /usr/lib/x86_64-linux-gnu/libgomp.so.1
>>>> > > >> (0x00007f69660cd000)
>>>> > > >>
>>>> > > >> *Performance*
>>>> > > >>
>>>> > > >> The only performance data related to OpenMP in MXNet I
was able
>>>> to
>>>> > find
>>>> > > is
>>>> > > >> here:
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://github.com/apache/incubator-mxnet/issues/9744#issuecomment-367711172
>>>> > > >>
>>>> > > >> Which in my understanding is testing imact of different
>>>> environment
>>>> > > >> variables for the same setup (using same bundled OpenMP
library).
>>>> > > >>
>>>> > > >> The libraries may differ in implementation and the Thread
>>>> Affinity
>>>> > > >> Interface [5] may have significant impact on performance.
>>>> > > >>
>>>> > > >> All compliers support it:
>>>> > > >>
>>>> > > >> - Clang / KMP_AFFINITY
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://github.com/clang-ykt/openmp/blob/master/runtime/src/kmp_affinity.cpp
>>>> > > >>
>>>> > > >> - GCC / GOMP_CPU_AFFINITY
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://gcc.gnu.org/onlinedocs/gcc-4.7.1/libgomp/GOMP_005fCPU_005fAFFINITY.html
>>>> > > >>
>>>> > > >> - Intel / KMP_AFFINITY
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://software.intel.com/en-us/node/522689#6E24682E-F411-4AE3-A04D-ECD81C7008D1
>>>> > > >>
>>>> > > >> - Visual Studio / SetThreadAffinityMask
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://docs.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-setthreadaffinitymask
>>>> > > >>
>>>> > > >> *Issues*
>>>> > > >>
>>>> > > >> Failed OpenMP assertion when loading MXNet compiled with
DEBUG=1
>>>> > > >> https://github.com/apache/incubator-mxnet/issues/10856
>>>> > > >>
>>>> > > >> libomp.so dependency (need REAL fix)
>>>> > > >> https://github.com/apache/incubator-mxnet/issues/11417
>>>> > > >>
>>>> > > >> mxnet-mkl (v0.12.0) crash when using (conda-installed)
numpy
>>>> with MKL
>>>> > > >> https://github.com/apache/incubator-mxnet/issues/8532
>>>> > > >>
>>>> > > >> Performance regression when OMP_NUM_THREADS environment
variable
>>>> is
>>>> > not
>>>> > > >> set
>>>> > > >> https://github.com/apache/incubator-mxnet/issues/9744
>>>> > > >>
>>>> > > >> Poor concat CPU performance on CUDA builds
>>>> > > >> https://github.com/apache/incubator-mxnet/issues/11905
>>>> > > >>
>>>> > > >> I would appreciate hearing your thoughts.
>>>> > > >>
>>>> > > >>
>>>> > > >> Best
>>>> > > >> Anton
>>>> > > >>
>>>> > > >> [1]
>>>> > > >>
>>>> > > >>
>>>> > >
>>>> >
>>>> https://github.com/apache/incubator-mxnet/blob/master/CMakeLists.txt#L400-L405
>>>> > > >> [2]
>>>> https://github.com/apache/incubator-mxnet/tree/master/3rdparty
>>>> > > >> [3]
>>>> https://github.com/intel/mkl-dnn/blame/master/README.md#L261-L265
>>>> > > >> [4]
>>>> https://github.com/intel/mkl-dnn/blame/master/README.md#L278-L280
>>>> > > >> [5] https://software.intel.com/en-us/node/522691
>>>> > > >>
>>>> > > >
>>>> > >
>>>> >
>>>>
>>>

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