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

Carlo Bertolli via cfe-dev cfe-dev at lists.llvm.org
Mon Mar 28 11:10:35 PDT 2016


Hi

Reading through the comments: both Chris and Chandler referenced to
liboffload, while I thought the subject of conversation was libomptarget
and SE.
I am being picky about names because liboffload is a library available as
part of omp (llvm's openmp runtime library) that, I believe, only targets
Intel Xeon Phi.

Did you mean liboffload or libomptarget?


Thanks

-- Carlo



From:	Alexandre Eichenberger via Openmp-dev
            <openmp-dev at lists.llvm.org>
To:	jhen at google.com
Cc:	llvm-dev at lists.llvm.org, cfe-dev at lists.llvm.org,
            openmp-dev at lists.llvm.org
Date:	03/28/2016 01:44 PM
Subject:	Re: [Openmp-dev] [cfe-dev] RFC: Proposing an LLVM subproject
            for parallelism runtime and support libraries
Sent by:	"Openmp-dev" <openmp-dev-bounces at lists.llvm.org>



Jason,

I concur with your decision since OMP and StreamExecutor fundamentally
differ in how dependences between consecutive tasks are expressed. OMP uses
task dependences to express constraint ordering between tasks that execute
on the host and/or on a particular device. Obviously, a stream is a DAG but
with very specific constraints (one linear ordering per stream), whereas
DAG generated by OMP dependences are arbitrary DAGs. This is not a jugement
statement, as in many ways stream are much more friendly to GPUs, it is
just a decision that the OMP and StreamExecutor "language experts" settled
on a different language expressivity/efficiency data point.

I read your blog on the similarities and differences with great interest. I
may venture to add another overlooked difference: OMP maps objects with
references counts (e.g. first time an object is mapped, its ref count is
zero, and the alloc on device and memory copy will occur; further nested
map will not generate any alloc and/or communication). In summary, OMP
primarily uses a dictionary of mapped variables to manage allocation and
data transfer, whereas StreamExecutor it appears to explicitly allocate and
move data.

Thanks for your work on this, much appreciated

Alexandre

-----------------------------------------------------------------------------------------------------

Alexandre Eichenberger, Master Inventor, Advanced Compiler Technologies
- research: compiler optimization (OpenMP, multithreading, SIMD)
- info: alexe at us.ibm.com http://www.research.ibm.com/people/a/alexe
- phone: 914-945-1812 (work) 914-312-3618 (cell)


 ----- Original message -----
 From: Jason Henline via Openmp-dev <openmp-dev at lists.llvm.org>
 Sent by: "Openmp-dev" <openmp-dev-bounces at lists.llvm.org>
 To: Andrey Bokhanko <andreybokhanko at gmail.com>, Chandler Carruth
 <chandlerc at google.com>
 Cc: llvm-dev <llvm-dev at lists.llvm.org>, cfe-dev <cfe-dev at lists.llvm.org>,
 "openmp-dev at lists.llvm.org" <openmp-dev at lists.llvm.org>
 Subject: Re: [Openmp-dev] [cfe-dev] RFC: Proposing an LLVM subproject for
 parallelism runtime and support libraries
 Date: Mon, Mar 28, 2016 12:38 PM

 I did a more thorough read through liboffload and wrote up a more detailed
 doc describing how StreamExecutor platforms relate to libomptarget RTL
 interfaces. The doc also describes why the lack of support for streams in
 libomptarget makes it impossible to implement some of the most important
 StreamExecutor platforms in terms of libomptarget (
 https://github.com/henline/streamexecutordoc/blob/master/se_and_openmp.rst
 ). When I was originally optimistic about using liboffload to implement
 StreamExecutor platforms, I was not aware of this issue with streams.
 Thanks to Carlo Bertolli for bringing this to my attention.

 After having looked in detail at the liboffload code, it sounds like the
 best thing to do at this point is to keep StreamExecutor and liboffload
 separate, but to leave the door open to implement future StreamExecutor
 platforms in terms of liboffload. From the recent messages on this subject
 from Carlo and Andrey it seems like there is a general consensus on this,
 so I would like to move forward with the StreamExecutor project in this
 spirit.

 On Tue, Mar 15, 2016 at 5:09 PM Jason Henline <jhen at google.com> wrote:
  I created a GitHub repo that contains the documentation I have been
  creating for StreamExecutor. https://github.com/henline/streamexecutordoc

  It contains the design docs from the original email in this thread, and
  it contains a new doc I just made that gives a more detailed sketch of
  the StreamExecutor platform plugin interface. This shows which methods
  must be implemented to support a new platform in StreamExecutor, or to
  provide a new implementation for an existing platform (e.g. using
  liboffload to implement the CUDA platform).

  I wrote up this doc in response to a lot of good questions I am getting
  about the details of how StreamExecutor might work with the code OpenMP
  already has in place.

  Best Regards,
  -Jason

  On Tue, Mar 15, 2016 at 12:28 PM Andrey Bokhanko <
  andreybokhanko at gmail.com> wrote:
    Hola Chandler,

    On Tue, Mar 15, 2016 at 1:44 PM, Chandler Carruth via Openmp-dev <
    openmp-dev at lists.llvm.org> wrote:
     It seems like if the OpenMP folks want to add a liboffload plugin to
     StreamExecutor, that would be an awesome additional platform, but I
     don't see why we need to force the coupling here.


    Let me give you a reason: while user-facing sides of StreamExecutor and
    OpenMP are quite different (and each warrants its place under the
    sun!), internal SE's offloading interface and liboffload are doing
    exactly the same thing. Why we want to duplicate code? As previous
    replies demonstrated, SE can't serve OpenMP's needs, while liboffload
    API seems to be general enough to serve SE well (though this has to be
    verified, of course -- as I understand, Jason is going to do this).

    Sure, there is no "must have need" to couple SE and liboffload, but
    this sounds like a solid software engineering decision to me. Or,
    quoting Jason, who said this much better than me:

    > Although OpenMP and StreamExecutor support different programming
    models,
    > some of the work they perform under the hood will likely be very
    similar.
    > By sharing code and domain expertise, both projects will be improved
    and
    > strengthened as their capabilities are expanded. The StreamExecutor
    > community looks forward to much collaboration and discussion with
    OpenMP
    > about the best places and ways to cooperate.

    Espere veure't demà!

    Yours,
    Andrey
    =====
    Enginyer de Software
    Intel Compiler Team

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