[Openmp-dev] Runtime error when executing multiple target regions within a target data region

[Jonas Hahnfeld via Openmp-dev]

Hi Joachim,

from internal discussion I know that you are targeting NVPTX and not 
x86_64 (aka "host offloading") - which is an important information in 
that case.

The arrays a and b are not mapped in the target region, so according to 
OpenMP 4.5, section 2.15.5 on page 215:
> A variable that is of type pointer is treated as if it had appeared in 
> a map clause as a zero-length array section.

However, Clang currently doesn't seem to do that; manually adding 
map(a[0:0], b[0:0]) leads to the expected output:
256, 992, 0
0
256, 992, 0
1
256, 992, 0
2
256, 992, 0
3
256, 992, 0
4
256, 992, 0
5
256, 992, 0
6
256, 992, 0
7
256, 992, 0
8
256, 992, 0
9
So the application has previously really been executing the fallback 
code on the host.
(Note that your commented combined directive works fine with the 
standalone data directives, because it actually has the map-clause.)

Follow-up questions:
1) Why doesn't the fallback printf as well?
2) Why does libomp have difficulties spawning teams after a while?

Cheers,
Jonas

On 2018-07-13 11:50, Joachim Protze via Openmp-dev wrote:
> Hi all,
> 
> we experience strange errors when we try to launch multiple target
> regions within a data region, see attached code. The result when using
> unstructured data mapping is similar. We are using clang built from
> trunk this week.
> 
> When we map the data for each iteration (as in line 23), the whole
> code runs through. When we use a larger value for TEAMS, the execution
> falls back to the host in an earlier iteration (for 1024 in the second
> iteration instead of 7th as shown below).
> 
> So, there seems to be an issue with the allocation of teams, when the
> data region stays open. Any ideas, how this can be fixed?
> 
> Best,
> Joachim
> 
> 
> Output when running the attached code (num_teams, thread_limit,
> is_initial_device):
> 
> 256, 992, 0
> 0
> 256, 992, 0
> 1
> 256, 992, 0
> 2
> 256, 992, 0
> 3
> 256, 992, 0
> 4
> 256, 992, 0
> 5
> 256, 992, 0
> OMP: Warning #96: Cannot form a team with 256 threads, using 48 
> instead.
> OMP: Hint Consider unsetting KMP_DEVICE_THREAD_LIMIT
> (KMP_ALL_THREADS), KMP_TEAMS_THREAD_LIMIT, and OMP_THREAD_LIMIT (if
> any are set).
> 48, 2147483647, 1
> 6
> 48, 2147483647, 1
> 7
> 48, 2147483647, 1
> 8
> 48, 2147483647, 1
> 9
> 
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