Bug#645052: kernel only recognizes 32G of memory
On Wed, 2011-10-12 at 14:11 +0100, Ben Hutchings wrote:
> On Wed, 2011-10-12 at 08:26 +0100, Ian Campbell wrote:
> > On Wed, 2011-10-12 at 08:46 +0300, Dmitry Musatov wrote:
> > > The config option XEN_MAX_DOMAIN_MEMORY controls how much memory a
> > > Xen instance is seeing. The default for 64bit is 32GB, which is the
> > > reason that m2.4xlarge Amazon EC2 instances only report this amount of
> > > memory.
> > > Please set this limit to 70GB as there is a known restriction for
> > > t1.micro instances at about 80GB.
> > > Similar bug exists and Ubuntu where it's already fixed
> > > (https://bugs.launchpad.net/ubuntu/+source/linux-ec2/+bug/667796)
> >
> > Is this the sort of change we can consider making in a stable update?
> > I'm not at all sure, although my gut feeling is that it would be safe.
> [...]
>
> I think so. But what is the trade-off? There must be some reason why
> this isn't set to however many TB the kernel can support.
It effects the amount of space set aside for the P2M table (the mapping
of physical to machine addresses). In the kernel in Squeeze this space
is statically reserved in BSS so increasing it will waste some more
memory, according to the Kconfig comment it is 1 page per GB.
In a more up to date kernel the space comes from BRK and is reclaimed if
it is not used, MAX_DOMAIN_MEMORY was bumped to default to 128G in the
same change.
Ian.
--
Ian Campbell
Current Noise: Devin Townsend - Namaste
The only way to learn a new programming language is by writing programs in it.
-- Brian Kernighan
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