On 2019/06/13 11:03, Jonathan Thornburg wrote: > 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" >
It's really performance _on OpenBSD_ that is of interest when deciding whether it's worth the ongoing maintenance to go down this path here :)
