Re: Memory management broken by "mm: reclaim small amounts of memory when an external fragmentation event occurs"

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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