On Mon, 2019-04-08 at 10:52 +0100, Mel Gorman wrote: > First, if pa-risc is !NUMA then why are separate local ranges > represented as separate nodes? Is it because of DISCONTIGMEM or > something else? DISCONTIGMEM is before my time so I'm not familiar > with it and I consider it "essentially dead" but the arch init code > seems to setup pgdats for each physical contiguous range so it's a > possibility. The most likely explanation is pa-risc does not have > hardware with addressing limitations smaller than the CPUs physical > address limits and it's possible to have more ranges than available > zones but clarification would be nice. Let me try, since I remember the ancient history. In the early days, there had to be a single mem_map array covering all of physical memory. Some pa-risc systems had huge gaps in the physical memory; I think one gap was somewhere around 1GB, so this lead us to wasting huge amounts of space in mem_map on non-existent memory. What CONFIG_DISCONTIGMEM did was allow you to represent this discontinuity on a non-NUMA system using numa nodes, so we effectively got one node per discontiguous range. It's hacky, but it worked. I thought we finally got converted to sparsemem by the NUMA people, but I can't find the commit. James
