How do people use these things? Map MPI ranks to dies, instead of compute nodes?
* It just seems like a lot of threads to wrangle without some hierarchy. Nested OpenMP is also possible…
* I’m wondering if explicit communication is better from one die to another in this sort of system.
The above doesn't even consider the possibility of multi-CPU systems. I suspect the existing programming models are quickly going to become insufficient for modeling these systems.
I also find myself wondering how atomic instruction performance will fare on these. GPU ISA and memory model on CPU when?
Before 8 cores per die (introduced in Zen 3, and retained in 4, 5 and 6), the Zen 1/+ and 2 series this would have been two sets of four cores instead of one set of eight (and a split L3 instead of a unified one). I can't remember if the split-CCX had its own NUMA layer in the tree or not, or if they were just iterated in pairs.
I realize that HPC code can be customized to the specific device it will be run on but more widely deployed software is going to want to abstract these increasingly complex relationships.
Ironically, a lot of people keep shooting themselves in the foot and blindly using MPI or OpenMP or any of the other popular industry supported frameworks, and then thinking that magically bails them out. It doesn't.
The most important thing you need, above all others: make sure the problem you're solving can be parallelized, and CPUs are the right way of doing it. Once you've answered this question, and the answer is yes, you can just write it pretty much normally.
Also, ironically, you can write Java that isn't shit and takes advantage of systems like these. Sun and the post-Sun community put a lot of work into the Hotspot JVM to make it scale alarmingly well on high core count machines. Java used correctly is performant.
Even today, I think very large enterprise systems (where a single kernel runs on a single system that spans multiple racks) are built like this, too.