Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

[Dave Hansen]

On Wed, 2005-11-02 at 20:02 +1100, Nick Piggin wrote:
> I agree. Especially considering that all this memory hotplug usage for
> hypervisors etc. is a relatively new thing with few of our userbase
> actually using it. I think a simple zones solution is the right way to
> go for now.

I agree enough on concept that I think we can go implement at least a
demonstration of how easy it is to perform.

There are a couple of implementation details that will require some
changes to the current zone model, however.  Perhaps you have some
suggestions on those.

In which zone do we place hot-added RAM?  I don't think answer can
simply be the HOTPLUGGABLE zone.  If you start with sufficiently small
of a machine, you'll degrade into the same horrible HIGHMEM behavior
that a 64GB ia32 machine has today, despite your architecture.  Think of
a machine that starts out with a size of 256MB and grows to 1TB.

So, if you have to add to NORMAL/DMA on the fly, how do you handle a
case where the new NORMAL/DMA ram is physically above
HIGHMEM/HOTPLUGGABLE?  Is there any other course than to make a zone
required to be able to span other zones, and be noncontiguous?  Would
that represent too much of a change to the current model?

>From where do we perform reclaim when we run out of a particular zone?
Getting reclaim rates of the HIGHMEM and NORMAL zones balanced has been
hard, and I worry that we never got it quite.  Introducing yet another
zone makes this harder.

Should we allow allocations for NORMAL to fall back into HOTPLUGGABLE in
any case?

-- Dave

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