Re: [PATCH 5/5] [hugetlb] Try to grow pool for MAP_SHARED mappings

Lee Schermerhorn
2007-07-18T14:44:49Z

On Tue, 2007-07-17 at 16:42 -0700, Nish Aravamudan wrote:
> On 7/13/07, Paul Jackson <pj@sgi.com> wrote:
> > Adam wrote:
> > > To be honest, I just don't think a global hugetlb pool and cpusets are
> > > compatible, period.
> >
> > It's not an easy fit, that's for sure ;).
>
> In the context of my patches to make the hugetlb pool's interleave
> work with memoryless nodes, I may have pseudo-solution for growing the
> pool while respecting cpusets.
>
> Essentially, given that GFP_THISNODE allocations stay on the node
> requested (which is the case after Christoph's set of memoryless node
> patches go in), we invoke:
>
> pol = mpol_new(MPOL_INTERLEAVE, &node_states[N_MEMORY])
>
> in the two callers of alloc_fresh_huge_page(pol) in hugetlb.c.
> alloc_fresh_huge_page() in turn invokes interleave_nodes(pol) so that
> we request hugepages in an interleaved fashion over all nodes with
> memory.
>
> Now, what I'm wondering is why interleave_nodes() is not cpuset aware?
> Or is it expected that the caller do the right thing with the policy
> beforehand? If so, I think I could just make those two callers do
>
> pol = mpol_new(MPOL_INTERLEAVE, cpuset_mems_allowed(current))
>
> ?
>
> Or am I way off here?

Nish:

I have always considered the huge page pool, as populated by
alloc_fresh_huge_page() in response to changes in nr_hugepages, to be a
system global resource. I think the system "does the right
thing"--well, almost--with Christoph's memoryless patches and your
hugetlb patches. Certaintly, the huge pages allocated at boot time,
based on the command line parameter, are system-wide. cpusets have not
been set up at that time.

It requires privilege to write to the nr_hugepages sysctl, so allowing
it to spread pages across all available nodes [with memory], regardless
of cpusets, makes sense to me. Altho' I don't expect many folks are
currently changing nr_hugepages from within a constrained cpuset, I
wouldn't want to see us change existing behavior, in this respect. Your
per node attributes will provide the mechanism to allocate different
numbers of hugepages for, e.g., nodes in cpusets that have applications
that need them.

Re: the "well, almost": nr_hugepages is still "broken" for me on some
of my platforms where the interleaved, dma-only pseudo-node contains
sufficient memory to satisfy a hugepage request. I'll end up with a few
hugepages consuming most of the dma memory. Consuming the dma isn't the
issue--there should be enough remaining for any dma needs. I just want
more control over what gets placed on the interleaved pseudo-node by
default. I think that Paul Mundt [added to cc list] has similar
concerns about default policies on the sh platforms. I have some ideas,
but I'm waiting for the memoryless nodes and your patches to stabilize
in the mm tree.