Re: [Bug-apl] Experimental OpenMP patch

[Juergen Sauermann]

Hi David,

looks good! Some comments, though.

1 .you could adapt src/testcases/Performance.pt with some longer
skalar functions in order to get some performance figures. You can start 
it like this:

./apl -T testcases/Performance.pt

2. I believe we should not bother the user with specifying 
parallelization parameters in ⎕SYL.
I would rather ./configure CORES=n with n=1 meaning no parallel 
execution, CORES=auto
being the number of cores on the build machine, and explicit numbers n>1 
meaning that
n cores shall be used. This would generate slightly faster code than 
computing array bounds
at runtime. Its a bit more hassle for the user, but may pay off soon.

3. Yes, GNU APL throws many exception (almost every APL error was thrown 
from somewhere),
 and I was excpecting that we have to catch them on the throwing 
processor. Not too difficult if
we do it on the top level.

4. It would be good to understand how the OPenMP loops work. I could 
imagined one of two strategies:

- in loop(j, MAX)   thread j executes iteration j, j+CORES, ...
- thread j executes iterations j*MAX/CORES ... (j+1)*MAX/CORES

The first strategy interleaves the data and is more intuitive
while the second uses blocks of data and is more cache-friendly and 
therefore probably
giving better performance.

5. Not sure if your earlier comment on letting the scheduler decide is 
correct. I have been doing
pthread programming in the past and I have seen cases where the 
scheduler fooled itself and
led to cases where the same problem took more than double the capacity 
compared to explicit
affinity on a 4-core CPU. I would expect that APL generates very 
fine-graned and short-lived
pieces of execution and the scheduler may not be optimized for that. I 
guess we have to try that out.

/// Jürgen



On 03/11/2014 08:02 AM, David B. Lamkins wrote:
> Juergen's suggestion prompted me to attempt an implementation using
> OpenMP rather than the by-hand coding that I had been anticipating.
> Attached is a quick-and-dirty patch to enable GNU APL to be build with
> OpenMP support.
>
> ./configure --with-openmp
>
> There are many rough edges, both in the Makefile and the code.
>
> --with-openmp would ideally check to see whether the compiler supports
> OpenMP. It may be necessary to check the compiler version, as different
> compilers support different versions of OpenMP. Also, I've assumed
> compilation on/for Linux despite the fact that GNU APL and OpenMP should
> be buildable with the right Windows compiler.
>
> As one might expect, OpenMP requires that any throw from a worker thread
> must be caught by the same thread. I'm almost certain that this
> restriction could be violated by GNU APL code as currently written.
>
> The good news, though, is that the changes are benign; in the absence of
> --with-openmp, GNU APL's behavior is unchanged.
>
> With OpenMP support, ⎕syl is extended to access some of OpenMPs
> parameters.
>
> I've done only trivial testing at this point; just enough to verify that
> compiling OpenMP support doesn't obviously break GNU APL.
>
> I haven't confirmed that the OpenMP #pragmas on the key loops in
> SkalarFunction.cc have any effect on execution time or processor core
> utilization. I hope to do more testing later this week.
>
> Best wishes,
>    David