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

Wed Jun 1 08:38:07 PDT 2016

----- 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 9:42:34 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 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?

Good question, although I'm under the impression that there are multiple developers.

> 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.

I think they could, but... SE has users, even if they're just inside of Google currently, and some of the relevant code is in a newly-popular open-source project (TensorFlow), and so I infer a certain amount of experience in SE's development team. Will my users want to use SE? I have no idea. I do care about OpenMP's offloading model because I'm certain I'll have users who care deeply about it within a couple of years. So regarding my optimistic attitude you describe below, I'm interested here is getting another set of eyes on the OpenMP offloading implementation, some sharing of expertise, voicing of alternate viewpoints, etc. -- hopefully, in the end, this will benefit both projects. That might influence, for the better, future versions of OpenMP. It might also end up providing a model that better serves (perhaps though another abstraction) my C++-based users for which OpenMP currently does not work well. None of this might happen, but it definitely won't happen unless we push for it. I don't think we get any of these other benefits it they just stick the code on github and develop it on their own. If they're willing to do this as part of the LLVM community, I think we should take advantage of that.

> >> > 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?

You're right. SE is certainly different from all of these, higher-level than some and lower-level than others. And, of course, I'm reminded of this: https://xkcd.com/927/

> 
> 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,

What they have is a potentially-valuable contribution to our project. For me, this is about the strategic health of parts of the project I care about. I have reasons (that I have not been subtle about), that I think there are potential benefits to having both SE and libompoffload together in some sense.

To be clear, I am an optimist. I think that different groups can collaborate, or at least share technical discussions, and mutually improve their work. It does not always happen, but we can encourage it by creating an environment in which it is more likely to occur.

Thanks again,
Hal

> but I don't see this as being fair. Nothing
> stops them from posting the code online somewhere. 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory