Re: [Patch1/4]: fake numa for x86_64 patch

[Mel Gorman]

On Tue, 28 Nov 2006, Rohit Seth wrote:

> On Tue, 2006-11-28 at 13:24 +0000, Mel Gorman wrote:
> > On Mon, 27 Nov 2006, Rohit Seth wrote:
> >
> > > Hi Mel,
> > >
> > > On Mon, 2006-11-27 at 13:18 +0000, Mel Gorman wrote:
> > > > On Wed, 22 Nov 2006, Rohit Seth wrote:
> > > >
> > > > > This patch provides a IO hole size in a given address range.
> > > >
> > > > Hi,
> > > >
> > > > This patch reintroduces a function that doubles up what
> > > > absent_pages_in_range(start_pfn, end_pfn). I recognise you do this because
> > > > you are interested in hole sizes before add_active_range() is called.
> > >
> > > Right.
> > >
> > > > However, what is not clear is why these patches are so specific to x86_64.
> > >
> > > Specifically in the fake numa case, we want to make sure that we don't
> > > carve fake nodes that only have IO holes in it.  Unlike the real NUMA
> > > case, here we don't have SRAT etc. to know the memory layout beforehand.
> > >
> > >
> > > > It looks possible to do the work of functions like split_nodes_equal() in
> > > > an architecture-independent manner using early_node_map rather than
> > > > dealing with the arch-specific nodes array. That would open the
> > > > possibility of providing fake nodes on more than one architecture in the
> > > > future.
> > >
> > > The functions like splti_nodes_equal etc. can be abstracted out to arch
> > > independent part.  I think the only API it needs from arch dependent
> > > part is to find out how much real RAM is present in range without have
> > > to first do add_active_range.
> >
> > That is a problem because the ranges must be registered with
> > add_active_range() to work out how much real RAM is present.
>
> Right.  And that is why I need e820_hole_size functionality. BTW, what
> is the concern in having that function?

Because it provides almost identical functionality to another function. If 
that can be avoided, it's preferable.

> > > Though as a first step, let us fix the x86_64 (as it doesn't boot when
> > > you have sizeable chunk of IO hole and nodes > 4).
> >
> > Ok.
> >
> > > I'm also not sure if other archs actually want to have this
> > > functionality.
> >
> > It's possible that the containers people are interested in the possibility
> > of setting up fake nodes as part of a memory controller.
> That is precisely why I'm doing it :-)
>
> > > > What I think can be done is that you register memory as normal and then
> > > > split up the nodes into fake nodes. This would remove the need for having
> > > > e820_hole_size() reintroduced.
> > >
> > > Are you saying first let the system find out real numa topology and then
> > > build fake numa on top of it?
> >
> > Yes, there is nothing stopping you altering the early_node_map[] before
> > free_area_init_node() initialises the node_mem_map. If you do hit a
> > problem, it'll be because x86_64 allocates it's own node_mem_map with
> > CONFIG_FLAT_NODE_MEM_MAP is set. Is that set when setting up fake nodes?
>
> I thought they both (real numa + fake numa) operate on same data
> structures. I'll have to double check.
>
> -rohit

--
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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