On 08.04.19 16:29, James Bottomley wrote: > 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, you tried once: https://patchwork.kernel.org/patch/729441/ It seems we better should move over to sparsemem now? Helge
