On Thu, Jun 04, 2020 at 08:55:19PM +0200, David Hildenbrand wrote:
> >> E.g., on powerpc that's 16MB so they have *a lot* of memory blocks.
> >> That's why that's not papering over the problem. Increasing the memory
> >> block size isn't always the answer.
> >
> > Ok. If you don't mind, what's the purpose of hotplugging at that granularity?
> > I'm simply curious.
>
> On bare metal: none with that big machines AFAIKS. :)
Sounds about right :)
> For VMs/partitions it gives you much more flexibility ("cloud", kata
> containers, memory overcommit, ...).
>
> Assume you have a VM with some initial memory size (e.g., 32GB). By
> hotplugging up to 256 DIMMs you cab grow in small steps (e.g., 128MB, up
> to 64GB, 256MB, up to 96GB, ...). And if you online all the memory
> blocks MOVABLE, you can shrink in these small steps.
Yeah, sorry for not being clear, I meant why does powerpc hotplug at "only" 16M.
> Regarding PowerPC, AFAIK it also gives the OS more flexibility to find
> memory blocks that can be offlined and unplugged, especially without the
> MOVABLE zone. Finding some scattered 16MB blocks that can be offlined is
> easier than finding one bigger (e.g., 2GB) memory block that can be
> offlined. And the history of powerpc dlpar dates back to pre-MOVABLE
> days (there is a paper from 2003).
Makes sense, thanks!
> I do think your change mostly affects bare metal where you do not care
> about hotplugging small memory blocks. Maybe an even better check would be
>
> if (!in_vm() {
> bz = MAX_BLOCK_SIZE;
> goto none;
> }
>
> because I doubt we have bare metal machines > 64 where we want to
> hot(un)plug DIMMs < 2G.
Yeah, agreed, not these days.
> But maybe there is a use case I am not aware of
> ... and I don't know an easy way to check whether we are running inside
> a VM or not (like kvm_para_available() ... ).
What about this? Works on bare metal and kvm, so presumably all the other HVs
too.
if (x86_hyper_type == X86_HYPER_NATIVE) {
bz = MAX_BLOCK_SIZE;
goto done;
}
