Re: [discuss] Re: [patch 3/3] add x86-64 support for memory hot-add

[Andi Kleen <ak@suse.de>]

On Fri, Dec 09, 2005 at 04:16:41PM -0800, Keith Mannthey wrote:
> > These should be all __cpuinit.
> >
> > In general SRAT has a hotplug memory bit so it's possible
> > to predict how much memory there will be in advance. Since
> > the overhead of the kernel page tables should be very
> > low I would prefer if you just used instead.
> 
> How much overhead would there be?

It's 2MB pages in 3levels, so roughly 3*8=24 bytes per 2MB or roughly
512 bytes per GB (rounded up always to the next page)

> 
> > (i.e. instead of extending the kernel mapping preallocate
> > the direct mapping and just clear the P bits)
> 
> On my box the SRAT for hot-add areas exposed are from the end
> installed memory to way out in outerspace.
> SRAT: hot plug zone found 280000000 - 2300000000
> I can't hot add that sort of range on my box but the bios didn't want
> to limit or is planing for really really big dimms.

You're just proving someone's (anyone want to volunteer their name? ;-]
I think Linus pointed it out originally, so let's call it Linus') 
law - as soon as we use some BIOS feature we soon find a BIOS 
that will get it totally wrong.

Anyways, I retracted anyways because of some other issues so Matt's 
original approach should be ok.

-Andi