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

Sergey Ostanevich via llvm-dev
Mon, 28 Mar 2016 21:31:17 -0000

Jason,

Am I got it right, that SE interfaces are bound to the stream that is
passed as argument? As I can see the stream is an abstraction of the target
- hence data transfers for particular stream is limited to this stream?
As for libomptarget implementation the data once offloaded can be reused in
all offload entries, without additional data transfer. Is it possible in SE
approach?

Regarding the kernels storing in memory or on file: the design was
originally to provide offload entries within the same object file as host
code. It is intended to ease adoption of the heterogeneous approach: there
should be no changes to build scripts. The resultant executable/library
obtained from the build should be self-contained and user will have no
extra problems with target objects/files availability at rutnime.

Sergos.


On Mon, Mar 28, 2016 at 9:47 PM, Jason Henline via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> 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
>>>
>>> _______________________________________________
>>> Openmp-dev mailing list
>>> Openmp-dev at lists.llvm.org
>>> *http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev*
>>> <http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev>
>>>
>>> _______________________________________________
>>> Openmp-dev mailing list
>>> Openmp-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev
>>>
>>>
>>>
>>>
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