[GRASS-dev] Debugging, parallelism, etc.

[Brad ReDacted]

Those variables would be...? Is this documented somewhere? If not, can 
we get it documented?

OpenMP can do far more than just loop "unrolling", these days. Tasking 
sections to run concurrent is also quite trivial. It can also offload to 
GPU, etc. Check out the v4.5+ spec. It's pretty impressive. I believe it 
can do most of what pthreads does, but you certainly lose control of 
implementation details. Some compilers have an omp library while others 
convert to pthreads.

I do find myself rewriting algorithms so that OpenMP can handle them. It 
doesn't seem to handle nested loops with breaks very well and I'm not 
entirely sure why.

On 10/9/2022 10:43 AM, William Hargrove wrote:
> Can still run GRASS outside the shell by setting all of the 
> environment variables appropriately ...
>
> OpenMP just works by "unrolling" all of the determinate loops, i.e., 
> the ones that iterate a fixed number of times.  No speedups to 
> anything else.
>
> Speedup from OpenMP will be limited, depending on the number of 
> determinate loops present, and how much of the load they represent.
>
> pthreads are totally flexible, but the programmer has to specify 
> everything, very carefully ...
>
> But pthreads can speed up lots of stuff outside of determinate loops ...
>
>
> HTH,
>
> Bill H.
>
>
> On 10/9/2022 12:37 PM, Brad ReDacted wrote:
>> Hello,
>>
>> I'm working on adding parallelism to modules, but debugging is 
>> turning out to be a logistical nightmare:
>>
>> Why do I not get any reporting from GCC option 
>> '-fsanitize=address|thread"?
>>
>> I am also having trouble getting the profiler to work properly inside 
>> GRASS (I assume due to shell?). The gmon.out file produced has no 
>> usable data.
>>
>> OpenMP is extremely poorly supported by most tools. valgrind with 
>> helgrind reports a lot of nonsense. I can't seem to get the Intel 
>> linux tools to work properly, either.
>>
>> BTW, we are supporting both pthreads and OpenMP. While this isn't an 
>> issue in most cases, there can be races and deadlocks if not handled 
>> properly. Pthreads aren't entirely portable. OpenMP is. However, 
>> pthreads gives us a more control. May I suggest using OpenMP for most 
>> modules and reserve Pthreads to libraries, etc? Or should we start 
>> moving away from pthreads?
>>
>> Any suggestions would be greatly appreciated!
>>
>>
>
-- 
Best Regards,
-Brad