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

Jason Henline via cfe-dev cfe-dev at lists.llvm.org
Mon Mar 28 11:47:44 PDT 2016


Alexandre,

Thanks for further shedding some light on the way OpenMP handles
dependencies between tasks. I'm sorry for leaving that out of my document,
it was just because I didn't know much about the way OpenMP handled its
workflows.

On Mon, Mar 28, 2016 at 11:43 AM Jason Henline <jhen at google.com> wrote:

> Hi Carlo,
>
> Thanks for helping to clarify this point about libomptarget vs liboffload,
> I have been getting confused about it myself. I think the open question
> concerns libomptarget not liboffload (others can correct me if I have
> misunderstood). My analysis from looking through the code was that
> libomptarget had some similarities with the platform support in SE, so I
> just wanted to consider how those two libraries compared. I didn't do a
> comparison with liboffload.
>
> On Mon, Mar 28, 2016 at 11:11 AM Carlo Bertolli <cbertol at us.ibm.com>
> wrote:
>
>> 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
>>
>> [image: Inactive hide details for Alexandre Eichenberger via Openmp-dev
>> ---03/28/2016 01:44:12 PM---Jason,]Alexandre Eichenberger via Openmp-dev
>> ---03/28/2016 01:44:12 PM---Jason,
>>
>> 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*
>> <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*
>> <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*
>>    <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* <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* <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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>>
>>
>>
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