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

Nish Aravamudan
2007-07-20T20:35:52Z

On 7/18/07, Paul Mundt <lethal@linux-sh.org> wrote:
> On Wed, Jul 18, 2007 at 12:02:03PM -0400, Lee Schermerhorn wrote:
> > On Wed, 2007-07-18 at 08:17 -0700, Nish Aravamudan wrote:
> > > On 7/18/07, Lee Schermerhorn <Lee.Schermerhorn@hp.com> wrote:
> > > > 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.
> > >
> > > I fully agree that hugepages are a global resource.
> > >
> > > > 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.
> > >
> > > The issue is that with Adam's patches, the hugepage pool will grow on
> > > demand, presuming the process owner's mlock limit is sufficiently
> > > high. If said process were running within a constrained cpuset, it
> > > seems slightly out-of-whack to allow it grow the pool on other nodes
> > > to satisfy the demand.
> >
> > Ah, I see. In that case, it might make sense to grow just for the
> > cpuset. A couple of things come to mind tho':
> >
> > 1) we might want a per cpuset control to enable/disable hugetlb pool
> > growth on demand, or to limit the max size of the pool--especially if
> > the memories are not exclusively owned by the cpuset. Otherwise,
> > non-privileged processes could grow the hugetlb pool in memories shared
> > with other cpusets [maybe the root cpuset?], thereby reducing the amount
> > of normal, managed pages available to the other cpusets. Probably want
> > such a control in the absense of cpusets as well, if on-demand hugetlb
> > pool growth is implemented.
> >
> I don't see that the two are mutually exclusive. Hugetlb pools have to be
> node-local anyways due to the varying distances, so perhaps the global
> resource thing is the wrong way to approach it. There are already hooks
> for spreading slab and page cache pages in cpusets, perhaps it makes
> sense to add a hugepage spread variant to balance across the constrained
> set?

I'm not sure I understand why you say "hugetlb pools"? There is no
plural in the kernel, there is only the global pool. Now, on NUMA
machines, yes, the pool is spread across nodes, but, well, that's just
because of where the memory is. We already spread out the allocation
of hugepages across all NUMA nodes (or will, once my patches go in).
And I think with my earlier suggestion (of just changing the
interleave mask used for those allocations to be cpuset-aware), that
we'd spread across the cpuset too, if there is one. Is that what you
mean by "spread variant"?

> nr_hugepages is likely something that should still be global, so the
> sum of the hugepages in the per-node pools don't exceed this value.

Yes, that is the case now. Well, mostly because they are always
incremented together or decremented together.

> It would be quite nice to have some way to have nodes opt-in to the sort
> of behaviour they're willing to tolerate. Some nodes are never going to
> tolerate spreading of any sort, hugepages, and so forth. Perhaps it makes
> more sense to have some flags in the pgdat where we can more strongly
> type the sort of behaviour the node is willing to put up with (or capable
> of supporting), at least in this case the nodes that explicitly can't
> cope are factored out before we even get to cpuset constraints (plus this
> gives us a hook for setting up the interleave nodes in both the system
> init and default policies). Thoughts?

I guess I don't understand which nodes you're talking about now? How
do you spread across any particular single node (how I read "Some
nodes are never going to tolerate spreading of any sort")? Or do you
mean that some cpusets aren't going to want to spread (interleave?).

Oh, are you trying to say that some nodes should be dropped from
interleave masks (explicitly excluded from all possible interleave
masks)? What kind of nodes would these be? We're doing something
similar to deal with memoryless nodes, perhaps it could be
generalized?

Thanks,
Nish