[nova] Mempage fun

Sahid Orentino Ferdjaoui sahid.ferdjaoui at canonical.com
Tue Jan 8 10:06:31 UTC 2019

On Mon, Jan 07, 2019 at 05:32:32PM +0000, Stephen Finucane wrote:
> We've been looking at a patch that landed some months ago and have
> spotted some issues:
> https://review.openstack.org/#/c/532168
> In summary, that patch is intended to make the memory check for
> instances memory pagesize aware. The logic it introduces looks
> something like this:
>    If the instance requests a specific pagesize
>       (#1) Check if each host cell can provide enough memory of the
>       pagesize requested for each instance cell
>    Otherwise
>       If the host has hugepages
>          (#2) Check if each host cell can provide enough memory of the
>          smallest pagesize available on the host for each instance cell
>       Otherwise
>          (#3) Check if each host cell can provide enough memory for
>          each instance cell, ignoring pagesizes
> This also has the side-effect of allowing instances with hugepages and
> instances with a NUMA topology but no hugepages to co-exist on the same
> host, because the latter will now be aware of hugepages and won't
> consume them. However, there are a couple of issues with this:
>    1. It breaks overcommit for instances without pagesize request
>       runningĀ on hosts with different pagesizes. This is because we don't
>       allow overcommit for hugepages, but case (#2) above means we are now
>       reusing the same functions previously used for actual hugepage
>       checks to check for regular 4k pages

I think that we should not accept any overcommit. Only instances with
an InstanceNUMATopology associated pass to this part of check. Such
instances want to use features like guest NUMA topology so their
memory mapped on host NUMA nodes or CPU pinning. Both cases are used
for performance reason and to avoid any cross memory latency.

>    2. It doesn't fix the issue when non-NUMA instances exist on the same
>       host as NUMA instances with hugepages. The non-NUMA instances don't
>       run through any of the code above, meaning they're still not
>       pagesize aware

That is an other issue. We report to the resource tracker all the
physical memory (small pages + hugepages allocated). The difficulty is
that we can't just change the virt driver to report only small
pages. Some instances wont be able to get scheduled. We should
basically change the resource tracker so it can take into account the
different kind of page memory.

But it's not really an issue since instances that use "NUMA features"
(in Nova world) should be isolated to an aggregate and not be mixed
with no-NUMA instances. The reason is simple no-NUMA instances do not
have boundaries and break rules of NUMA instances.

> We could probably fix issue (1) by modifying those hugepage functions
> we're using to allow overcommit via a flag that we pass for case (#2).
> We can mitigate issue (2) by advising operators to split hosts into
> aggregates for 'hw:mem_page_size' set or unset (in addition to
> 'hw:cpu_policy' set to dedicated or shared/unset). I need to check but
> I think this may be the case in some docs (sean-k-mooney said Intel
> used to do this. I don't know about Red Hat's docs or upstream). In
> addition, we did actually called that out in the original spec:
> https://specs.openstack.org/openstack/nova-specs/specs/juno/approved/virt-driver-large-pages.html#other-deployer-impact
> However, if we're doing that for non-NUMA instances, one would have to
> question why the patch is necessary/acceptable for NUMA instances. For
> what it's worth, a longer fix would be to start tracking hugepages in a
> non-NUMA aware way too but that's a lot more work and doesn't fix the
> issue now.
> As such, my question is this: should be look at fixing issue (1) and
> documenting issue (2), or should we revert the thing wholesale until we
> work on a solution that could e.g. let us track hugepages via placement
> and resolve issue (2) too.
> Thoughts?
> Stephen

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