Re: discontiguous memory platforms

Ralf Baechle
2002-05-02T00:51:47Z

On Wed, May 01, 2002 at 04:23:43PM -0700, Jesse Barnes wrote:

> On Thu, May 02, 2002 at 01:17:50AM +0200, Andrea Arcangeli wrote:
> > so ia64 is one of those archs with a ram layout with huge holes in the
> > middle of the ram of the nodes? I'd be curious to know what's the
>
> Well, our ia64 platform is at least, but I think there are others.
>
> > hardware advantage of designing the ram layout in such a way, compared
> > to all other numa archs that I deal with. Also if you know other archs
> > with huge holes in the middle of the ram of the nodes I'd be curious to
> > know about them too. thanks for the interesting info!
>
> AFAIK, some MIPS platforms (both NUMA and non-NUMA) have memory
> layouts like this too. I've never done hardware design before, so I'm
> not sure if there's a good reason for such layouts. Ralf or Daniel
> might be able to shed some more light on that...

Just to give a few examples of memory layouts on MIPS systems. Sibyte 1250
is as follows:

- 256MB at physical address 0
- 512MB at physical address 0x80000000
- 256MB at physical address 0xc0000000
- The entire rest of the memory is mapped contiguously from physical
address 0x1:00000000 up.
All available memory is mapped from the lowest address up.

Origin 200/2000. Each node has an address space of 2GB, each node has 4
memory banks, that is each bank takes 512MB of address space. Even
unpopulated or partially populated banks take the full 512MB address
space. Memory in partially populated banks is mapped at the beginning
of the bank's address space; each node must have have at least one
bank with memory in it, that is something like

- 32MB @ physical address 0x00:00000000
- 32MB @ physical address 0x00:80000000
- 32MB @ physical address 0x01:00000000
...
- 32MB @ physical address 0x7f:00000000

would be a valid configuration. That's 8GB of RAM scattered in tiny
chunks of just 32mb throughout 256MB address space. In theory nodes
might not even have to exist, so

- 32MB @ physical address 0x00:00000000
- 32MB @ physical address 0x7f:00000000

would be a valid configuration as well.

There are other examples more but #1 is becoming a widespread chip and #2
is a rather extreme example just to show how far discontiguity may go.

Ralf