[nova] Mempage fun

Sean Mooney smooney at redhat.com
Tue Jan 8 13:39:16 UTC 2019

On Tue, 2019-01-08 at 11:06 +0100, Sahid Orentino Ferdjaoui wrote:
> 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
> >       running 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.
that is not nessisarialy correct.
if i request cpu pinning that does not imply that i dont want the ablitiy 
memory over subsricption. that is an artifact of how we chose to implement pinning
in the libvirt driver. for the case of cpu pinning specifically i have alway
felt it is wrong that we create a numa toplogy for the geust implicitly.
in the case of hw:numa_nodes=1 in the absence of any other extra spec or
image metadata i also do not think it is correct ot disabel over subsription
retroativly after supportin it for several years. requesting a numa toplogy
out side of requesting huge pages explcitly shoudl never have disabled over subsription
and changing that behavior should have both required a microvirtion and a nova spec.
https://review.openstack.org/#/c/532168 was simply a bug fix and therefor shoudl not
have changed the meaning of requesting a numa topology.

> >    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.
it is true that we should partion deployment useing host aggreates for
host with numa instance today and host that have non numa instances.
the reason issue 2 was raised is that the commit message implied that the patch
addressed mixing numa and non numa guest on the same host.

"Also when no pagesize is requested we should consider to compute memory usage based on small pages since the amount of
physical memory available may also include some large pages."

but the logic in the patch does not actully get triggered when the guest does not have numa topology so
it does not actully consider the total number of small page in that case.
this was linked to a down bugzilla you filed https://bugzilla.redhat.com/show_bug.cgi?id=1625119
and another for osp 10 https://bugzilla.redhat.com/show_bug.cgi?id=1519540
which has 3 costomer cases associated with it that downstream bugs. on closer
inspection of the patch dose not address the downstream bug at all as it
is expressly stating that nova does not consider small pages when mem_page_size is not
set but since we dont execute this code of non numa guest we dont actully resovle the issue.

when we consider the costomer cases associated with this specifcally the 3 the donwstream
bug claims to resove are
1.) a sheudler race where two pinned instances get shduled to the same set of resouces (this can only be fixed with
2.) mixing hugepage and non huge page guest reulted in OOM events
3.) instnace with a numa toplogy nolonger respect ram allocation ration.

the third customer issue was directly caused but backporting this patach.
the second issue would be resoved by using host aggreates  to segerage hugepage host from non numa hosts
and the first cant be addressed without premtivly claiming cpu/hugepages in the schduelr/placement.

> > 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:
> > 
> > 
> > 
> > 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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