In message <https://marc.info/?l=openbsd-ports&m=155992865732530&w=1>,
j () bitminer ! ca wrote
[[about openmp]]
> For long calculations, I have seen 2 cores with OpenMP take half the time
> as one core with/without. So yes, it works. But Amdahl's Law applies.
> If a calculation is long, and 20% is serial, and 80% parallelizable,
> then the runtimes will be 100%, 60%, 46% and 40% at 1, 2, 3 and 4 cores.
> The difference between 3 and 4 cores is not much. If 100% takes 4 hours,
> then the difference between 1 and 2 cores is significant.
I have a code which typically gets a speedup of ~6 using OpenMP on
8 cores and ~12 on 16 cores. Using OpenMP the code runs for anywhere
from a few days to a few weeks, so the OpenMP speedup is very significant.
The code is ~100K lines of C++, developed on OpenBSD, running on a
Linux supercomputer. I added OpenMP support in 2015, so it's all guarded
with an #ifdef which is disabled on OpenBSD. (Debugging the OpenMP
directly on the supercomputer was slightly painful; fortunately this
code's use of OpenMP is very simple, with only 3 parallel loops and
one per-thread data structure in the entire code.)
ciao,
--
-- "Jonathan Thornburg [remove -animal to reply]"
<jth...@astro.indiana-zebra.edu>
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
currently on the west coast of Canada
"There was of course no way of knowing whether you were being watched
at any given moment. How often, or on what system, the Thought Police
plugged in on any individual wire was guesswork. It was even conceivable
that they watched everybody all the time." -- George Orwell, "1984"
