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

C Bergström via cfe-dev cfe-dev at lists.llvm.org
Mon Mar 14 10:14:27 PDT 2016


/* ignorable rant */
I've publicly advocated it shouldn't have been there in the 1st place.
I have been quite vocal the work wasn't for everyone else to pay, but
should have been part of the initial design. (Basically getting it
right the 1st time - instead of forcing someone else to wade through a
bunch of cmake)

On Tue, Mar 15, 2016 at 1:10 AM, Cownie, James H
<james.h.cownie at intel.com> wrote:
>> I'd support some of Jame's comments if liboffload wasn't glued to OMP as it is now.
>
> I certainly have no objection to moving liboffload elsewhere if that makes it more useful to people.
> There is no real "glue" holding it there; it simply ended up in the OpenMP directory structure because that
> was an easy place to put it, not because that's the optimal place for it.
>
> To some extent it has stayed there because no-one has put in any effort to do the work to move it.
>
> -- Jim
>
> James Cownie <james.h.cownie at intel.com>
> SSG/DPD/TCAR (Technical Computing, Analyzers and Runtimes)
> Tel: +44 117 9071438
>
> -----Original Message-----
> From: C Bergström [mailto:cbergstrom at pathscale.com]
> Sent: Monday, March 14, 2016 5:01 PM
> To: Cownie, James H <james.h.cownie at intel.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>; cfe-dev <cfe-dev at lists.llvm.org>; openmp-dev at lists.llvm.org
> Subject: Re: [cfe-dev] RFC: Proposing an LLVM subproject for parallelism runtime and support libraries
>
> I'd support some of Jame's comments if liboffload wasn't glued to OMP
> as it is now. My attempts to decouple it into something with better
> design layering and outside of OMP source repo, have failed. For it to
> be advocated as "the" offload lib - it needs a home (imnsho) outside
> of OMP. Somewhere that others can easily play with it and not pay the
> OMP tax. It may tick some of the boxes which have been mentioned, but
> I'm curious how well it does when put under real workloads.
>
> On Tue, Mar 15, 2016 at 12:53 AM, Cownie, James H via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>> Jason,
>>
>>
>>
>> It’s great that Google are interested in contributing to the development of
>> LLVM in this area, and that you have code to support offload.
>>
>> However, I’m not sure that all of it is needed, since LLVM already has the
>> offload library which has been being developed in the context of OpenMP, but
>> actually provides a general facility. It has been a part of LLVM since April
>> 2014, and is already being used to offload to both Intel Xeon Phi and (at
>> least NVidia) GPUs. (The IBM folks can tell you more about that!)
>>
>>
>>
>> The main difference I see (at a very first glance!) is that your
>> StreamExecutor interfaces seem to be aimed more at end user code, whereas
>> the interface to the existing offload library has not been designed for the
>> user, but to be an interface from the compiler. That has advantages and
>> disadvantages
>>
>> Advantages:
>>
>> ·         It is a C level interface, so is callable from C,C++ and Fortran
>>
>> Disadvantages:
>>
>> ·         Using it directly from C++ user code may be harder than using
>> StreamExecutor.
>>
>>
>>
>> However, there is nothing in the interface that prevents it from being used
>> with CUDA or OpenCL, and it already seems to support the low level features
>> you cited as StreamExecutor’s advantages, though not the “looks just like
>> CUDA” aspects, since it’s explicitly vendor neutral.
>>
>>
>>
>>> StreamExecutor:
>>
>>>
>>
>>> * abstracts the underlying accelerator platform (avoids locking you into a
>>
>>> single vendor, and lets you write code without thinking about which
>>
>>> platform you'll be running on).
>>
>> Liboffload does this (and has a specific design for how to abstract new
>> devices and support them using device specific libraries).
>>
>>> * provides an open-source alternative to the CUDA runtime library.
>>
>> I am not a CUDA expert, so I can’t comment on this! As before, IBM should
>> comment.
>>
>>> * gives users a stream management model whose terminology matches that of
>>> the CUDA programming model.
>>
>> This is not abstract, but seems CUDA target specific, which is, if anything,
>> worrying for a supposedly vendor-neutral interface!
>>
>>> * makes use of modern C++ to create a safe, efficient, easy-to-use
>>> programming interface.
>>
>> No, because liboffload is an implementation layer, not intended to be
>> user-visible.
>>
>>
>>
>>> StreamExecutor makes it easy to:
>>
>>>
>>
>>> * move data between host and accelerator (and also between peer
>>> accelerators).
>>
>> Liboffload supports this.
>>
>>> * execute data-parallel kernels written in the OpenCL or CUDA kernel
>>> languages.
>>
>> I believe this should be easy; IBM can comment better, since they have been
>> working on GPU support.
>>
>>> * inspect the capabilities of a GPU-like device at runtime.
>>
>>> * manage multiple devices.
>>
>> Liboffload supports this.
>>
>>
>>
>> We’d therefore be very interested in seeing an approach that implemented a
>> C++ specific user-friendly interface on top of the existing liboffload
>> functionality, but we don’t see a reason to rework the OpenMP implementation
>> to use StreamExecutor (since what LLVM already has is working fine, and
>> supporting offload to both GPUs and Xeon Phi).
>>
>>
>>
>> -- Jim
>>
>> James Cownie <james.h.cownie at intel.com>
>> SSG/DPD/TCAR (Technical Computing, Analyzers and Runtimes)
>>
>> Tel: +44 117 9071438
>>
>>
>>
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