[llvm-dev] [Openmp-dev] [cfe-dev] RFC: Proposing an LLVM subproject for parallelism runtime and support libraries

Wed Jun 1 08:21:16 PDT 2016

> On Jun 1, 2016, at 7:42 AM, C Bergström via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On Wed, Jun 1, 2016 at 8:52 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>> ----- Original Message -----
>>> From: "C Bergström" <cbergstrom at pathscale.com>
>>> To: "Hal Finkel" <hfinkel at anl.gov>
>>> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "cfe-dev" <cfe-dev at lists.llvm.org>, "openmp-dev"
>>> <openmp-dev at lists.llvm.org>, "Chandler Carruth" <chandlerc at gmail.com>, "Carlo Bertolli" <cbertol at us.ibm.com>,
>>> "Andrey Bokhanko" <andreybokhanko at gmail.com>
>>> Sent: Wednesday, June 1, 2016 6:46:57 AM
>>> Subject: Re: [Openmp-dev] [llvm-dev] [cfe-dev] RFC: Proposing an LLVM subproject for parallelism runtime and support
>>> libraries
>>> 
>>> On Wed, Jun 1, 2016 at 7:22 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>>> 
>>>> ________________________________
>>>> 
>>>> From: "C Bergström" <cbergstrom at pathscale.com>
>>>> To: "Hal Finkel" <hfinkel at anl.gov>
>>>> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "cfe-dev"
>>>> <cfe-dev at lists.llvm.org>, "openmp-dev" <openmp-dev at lists.llvm.org>,
>>>> "Chandler Carruth" <chandlerc at gmail.com>, "Carlo Bertolli"
>>>> <cbertol at us.ibm.com>, "Andrey Bokhanko" <andreybokhanko at gmail.com>
>>>> Sent: Wednesday, June 1, 2016 12:19:19 AM
>>>> Subject: Re: [Openmp-dev] [llvm-dev] [cfe-dev] RFC: Proposing an
>>>> LLVM
>>>> subproject for parallelism runtime and support libraries
>>>> 
>>>> The thread has lost focus and cherry picking replies..
>>>> 
>>>> To restate things since maybe you missed my points
>>>> ----
>>>> 1) SE is a programming model and needs a home of it's own. Having a
>>>> programming model with it's headers and all other stuff glued into
>>>> a runtime
>>>> project which intends to be universal and PM agnostic doesn't make
>>>> sense.
>>>> 
>>>> They'd start in separate subdirectories.
>>>> 
>>>> 
>>>> 1.1) The more I look, the most it seems SE is just a step-child
>>>> project and
>>>> stuffing it in llvm while still not having any users or strong
>>>> backing
>>>> doesn't make sense. We have enough PM already and my gut feeling is
>>>> this
>>>> isn't going in a direction to bring in other stakeholders.
>>> 
>>> Point by point because I don't agree with what you write below
>>> entirely
>>> 
>>>> 
>>>> I think this is the core of my reply. OpenMP has a strong user
>>>> community,
>>> 
>>> I'd agree here
>>> 
>>>> but OpenMP 4 offloading is still young.
>>> 
>>> ditto
>>> 
>>>> OpenMP 4 offloading does not yet
>>>> have a real user community
>>> 
>>> agreement
>>> 
>>>> yet because the first implementations just
>>>> started shipping very recently.
>>> 
>>> Partially strongly disagree - Maybe you meant on Power8?
>>> Intel has had their support for OMP4 for quite some time. It may have
>>> been buggy and focused on simd directive, but as best I can tell they
>>> have worked quite hard to fix bugs and improve it.
>> 
>> I meant across many platforms. Intel had preliminary support for OpenMP 4 offloading directives even before OpenMP 4 was standardized (in 2013). This did not include all of what ended up in the standard, and the offloading part of the standard needed to be fixed in a breaking way for OpenMP 4.1/4.5 regardless, so this is certainly the exception rather than the rule.
>> 
>>> 
>>> (Shameless self promotion) We have had some degree of OMP4 offloading
>>> for like 1.5 years now.. It's mostly targeting the GPU and x86, but
>>> also more recently working on Power8/AArch64.
>>> 
>>> I'm quite certain these companies all have varying degrees of OMP4
>>> done: Cray, IBM, PGI
>> 
>> Yes, many are working on implementations, and some have shipped. There's a list here: http://openmp.org/wp/openmp-compilers/ - no one here really lists any OpenMP 4 offloading implementations before 2015. PGI does not currently list OpenMP 4 at all (although they've certainly done a lot of work on OpenACC).
>> 
>>> 
>>>> Furthermore, our implementation is certainly
>>>> quite new, and OpenMP 4 offloading is really quite akin to SE in
>>>> that
>>>> regard.
>>> 
>>> Strongly disagree - Intel has been working with the LLVM community on
>>> the parsing/sema and other parts of OMP4 for like a year or more..
>>> This has been a long and well vetted process backed back a well
>>> defined standard.
>> 
>> Yes, both Intel and IBM have contributed significantly to Clang in this regard.
>> 
>>> Compared to SE which is just some thing that popped
>>> up out of nowhere and
>> 
>> It popped up in TensorFlow, which itself popped up out of nowhere, but is already a popular open-source project for machine learning.
>> 
>>> has a couple people from Google throwing their
>>> weight around.
>> 
>> Google is a trusted member of the LLVM community and a major contributor. So I agree with you in the following sense: When Google promises to update the code to follow LLVM's coding conventions and to manage its future development in accordance with our community norms, I believe them. I think they've earned a place in the "trust but verify" category in this regard, and I think this is a positive thing.
>> 
> 
> yes, but not every project that is started is finished and who would
> be the maintainer if the person leaves Google?

Like anything that bitrot and is no longer maintained in LLVM: it gets deleted (or the sources stay there but it is no longer updated/maintained).
There are precedents: http://llvm.org/svn/llvm-project/

> I still don't see why
> they can't fork it on github or create a project to let it bake and
> get some users or traction.

The intent may be that instead of creating a separate community of users, it'd create its community inside the llvm projects community. That looks like a nice "feature" long term.

Cost/benefit: cost is not really existent, potential benefits are interesting.

-- 
Mehdi

> 
>>>> I view them both as experimental projects, and both have strong
>>>> backing with significant investment, so I expect both to mature
>>>> over time.
>>> 
>>> I'd agree both are experimental and probably need some work. I'd
>>> strongly disagree the amount of investment of both projects is equal.
>>> Again - OMP4 may have zero real world users, but there's a lot of
>>> stubborn people trying to make it work. Compared to SE which the
>>> people involved have yet to even answer basic things - like what's
>>> the
>>> future of SE and how to contribute to it.
>> 
>> They might not know what the future is. They have code that is useful to them, and they feel would be useful to others. There's strong potential for compiler integration in the future, but it is just a library now. They've talked about extending it to cover different backends (OpenCL, etc.), and investigating host-based-task dependencies, etc. My impression is that, in general, they'd like the code's users, present and future, to shape its future. All of this seems perfectly healthy. If it becomes part of LLVM, how to contribute to it will become clear.
> 
> I love your optimistic attitude, but the reality is that successful
> programming models take a significant amount of work. I'm well aware
> that they can handle at least part of that, but so far I'm not sure
> who else will invest in it..
> 
> The industry has : CUDA, OpenCL, OpenMP4, OpenACC, C++AMP, Parallel
> STL(??), some research projects from DOE and dead stuff like HMPP...
> Are all these so broken that they can't be fixed to fill the same
> necessity as SE?
> 
> How similar is it to the failed AMD Stream Computing (sorry AMD)
> http://www.ele.uri.edu/courses/ele408/StreamGPU.pdf
> 
> Intel's work had to live and grow outside for quite a while - Google
> may have more good karma, but I don't see this as being fair. Nothing
> stops them from posting the code online somewhere.
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