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

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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




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