On Mon, 4 Dec 2006, Andrew Morton wrote:
> My concern is that __GFP_MOVABLE is useful for fragmentation-avoidance, but
> useless for memory hot-unplug. So that if/when hot-unplug comes along
> we'll add more gunk which is a somewhat-superset of the GFP_MOVABLE
> infrastructure, hence we didn't need the GFP_MOVABLE code. Or something.
It is useless for memory unplug until we implement limits for unmovable
pages in a zone (per MA_ORDER area? That would fit nicely into the anti
frag scheme) or until we have logic that makes !GFP_MOVABLE allocations
fall back to a node that is not removable.
> That depends on how we do hot-unplug, if we do it. I continue to suspect
> that it'll be done via memory zones: effectively by resurrecting
> GFP_HIGHMEM. In which case there's little overlap with anti-frag. (btw, I
> have a suspicion that the most important application of memory hot-unplug
> will be power management: destructively turning off DIMMs).
There are numerous other uses as well (besides DIMM and node unplug):
1. Faulty DIMM isolation
2. Virtual memory managers can reduce memory without resorting to
balloons.
3. Physical removal and exchange of memory while a system is running
(Likely necessary to complement hotplug cpu, cpus usually come
with memory).
The multi zone approach does not work with NUMA. NUMA only supports a
single zone for memory policy control etc. Also multiple zones carry with
it a management overhead that is unnecessary for the MOVABLE/UNMOVABLE
distinction.
> perhaps not for the hugetlbpage problem. Whereas anti-fragmentation adds
> vastly more code, but can address both problems? Or something.
I'd favor adding full defragmentation.
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