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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Dave Hansen wrote:

> What the fragmentation patches _can_ give us is the ability to have 100%
> success in removing certain areas: the "user-reclaimable" areas
> referenced in the patch.  This gives a customer at least the ability to
> plan for how dynamically reconfigurable a system should be.

But the "user-reclaimable" areas can still be taken over by other
areas which become fragmented.

That's like saying we can already guarantee 100% success in removing
areas that are unfragmented and free, or freeable.

> After these patches, the next logical steps are to increase the
> knowledge that the slabs have about fragmentation, and to teach some of
> the shrinkers about fragmentation.

I don't like all this work and complexity and overheads going into a
partial solution.

Look: if you have to guarantee memory can be shrunk, set aside a zone
for it (that only fills with user reclaimable areas). This is better
than the current frag patches because it will give you the 100%
guarantee that you need (provided we have page migration to move mlocked
pages).

If you don't need a guarantee, then our current, simple system does the
job perfectly.

> After that, we'll need some kind of virtual remapping, breaking the 1:1
> kernel virtual mapping, so that the most problematic pages can be
> remapped.  These pages would retain their virtual address, but getting a
> new physical.  However, this is quite far down the road and will require
> some serious evaluation because it impacts how normal devices are able
> to to DMA.  The ppc64 proprietary hypervisor has features to work around
> these issues, and any new hypervisors wishing to support partition
> memory hotplug would likely have to follow suit.

I would more like to see something like this happen (provided it was
nicely abstracted away and could be CONFIGed out for the 99.999% of
users who don't need the overhead or complexity).

--
SUSE Labs, Novell Inc.

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