DOC: design-thoughts: commit numa-auto.txt
Lots of collected data and observations aggregated into a single commit so as not to lose them. Some parts below come from several commit messages and are incremental. Add captures and analysis of intel 14900 where it's not easy to draw the line between the desired P and E cores. The 14900 raises some questions (imagine a dual-die variant in multi-socket). That's the start of an algorithmic distribution of performance cores into thread groups. cpu-map currently conflicts a lot with the choices after auto-detection but it doesn't have to. The problem is the inability to configure the threads for the whole process like taskset does. By offering this ability we can also start to designate groups of CPUs symbolically (package, die, ccx, cores, smt). It can also be useful to exploit the info from cpuinfo that is not available in /sys, such as the model number. At least on arm, higher numbers indicate bigger cores and can be useful to distinguish cores inside a cluster. It will not indicate big vs medium ones of the same type (e.g. a78 3.0 vs 2.4 GHz) but can still be effective at identifying the efficient ones. In short, infos such as cluster ID not always reliable, and are local to the package. die_id as well. die number is not reported here but should definitely be used, as a higher priority than L3. We're still missing a discriminant between the l3 and cluster number in order to address heterogenous CPUs (e.g. intel 14900), though in terms of locality that's currently done correctly. CPU selection is also a full topic, and some thoughts were noted regarding sorting by perf vs locality so as never to mix inter- socket CPUs due to sorting. The proposed cpu-selection cannot work as-is, because it acts both on restriction and preference, and these two are not actions but a sequence. First restrictions must be enforced, and second the remaining CPUs are sorted according to the preferred criterion, and a number of threads are selected. Currently we refine the OS-exposed cluster number but it's not correct as we can end up with something poorly numbered. We need to respect the LLC in any case so let's explain the approach.
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