[Openmp-dev] Malloc in target region

[Jeff Hammond via Openmp-dev]

Does OpenMP 5 allow calling symbols that are not omp-declare-target?

In any case, I am concerned about malloc interception, having seen that
break many times in the past.  OpenMP isn't the only thing that wants to
intercept malloc, and nested interception may not be stable.

Why is omp_alloc not the best option here?  That avoids essentially all of
the issues I can think of.

Jeff

On Wed, Jun 24, 2020 at 11:12 AM Hal Finkel via Openmp-dev <
openmp-dev at lists.llvm.org> wrote:

> Hi, Jon,
>
> This is a great question.
>
> With reverse-offload support, and unified memory, we can support a model
> where memory allocation triggers reverse offload to the memory allocator on
> the host. In this mode, everything works as expected. We can, of course, do
> some static analysis and move allocations that don't escape to use some
> local allocation scheme, such as what we use without unified memory +
> reverse offload.
>
> Without such support, I think that "One heap per device + one for host.
> Each independent, pointers only valid on the thing that called malloc."
> makes the most sense. This also, as far as I know, matches what's available
> in CUDA today.
>
> "One heap per target offload region" doesn't make sense to me. One might
> clearly want to allocate in one target region, store the pointers in some
> data structure, and then access them in some other target region on the
> same device.
>
> Thanks again,
>
> Hal
> On 6/24/20 6:40 AM, Jon Chesterfield via Openmp-dev wrote:
>
> Hello OpenMP,
>
> Our language spec seems fairly light on what it means to call malloc from
> a target region.
>
> I can think of a few interpretations:
> - One heap per process. Malloc on target or host, free from either.
> Writable from either, or some other device. Might mean intercepting host
> libc. Convenient, slow.
> - One heap per device + one for host. Each independent, pointers only
> valid on the thing that called malloc.
> - One heap per target offload region, inaccessible from host.
> - Some other granularity.
>
> Generally gets faster as the restrictions increase.
>
> Anyone willing to state / guess what they or their users would expect?
> Bearing in mind that new is likely to call malloc and will gain the same
> properties.
>
> Thanks,
>
> Jon
>
>
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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