[openstack-dev] realtime kvm cpu affinities
henning.schild at siemens.com
Tue Jun 20 07:48:23 UTC 2017
We are using OpenStack for managing realtime guests. We modified
it and contributed to discussions on how to model the realtime
feature. More recent versions of OpenStack have support for realtime,
and there are a few proposals on how to improve that further.
But there is still no full answer on how to distribute threads across
host-cores. The vcpus are easy but for the emulation and io-threads
there are multiple options. I would like to collect the constraints
from a qemu/kvm perspective first, and than possibly influence the
I will put the summary/questions first, the text below provides more
context to where the questions come from.
- How do you distribute your threads when reaching the really low
cyclictest results in the guests? In  Rik talked about problems
like hold holder preemption, starvation etc. but not where/how to
schedule emulators and io
- Is it ok to put a vcpu and emulator thread on the same core as long as
the guest knows about it? Any funny behaving guest, not just Linux.
- Is it ok to make the emulators potentially slow by running them on
busy best-effort cores, or will they quickly be on the critical path
if you do more than just cyclictest? - our experience says we don't
need them reactive even with rt-networking involved
Our goal is to reach a high packing density of realtime VMs. Our
pragmatic first choice was to run all non-vcpu-threads on a shared set
of pcpus where we also run best-effort VMs and host load.
Now the OpenStack guys are not too happy with that because that is load
outside the assigned resources, which leads to quota and accounting
So the current OpenStack model is to run those threads next to one
or more vcpu-threads.  You will need to remember that the vcpus in
question should not be your rt-cpus in the guest. I.e. if vcpu0 shares
its pcpu with the hypervisor noise your preemptrt-guest would use
Is that kind of sharing a pcpu really a good idea? I could imagine
things like smp housekeeping (cache invalidation etc.) to eventually
cause vcpu1 having to wait for the emulator stuck in IO.
Or maybe a busy polling vcpu0 starving its own emulator causing high
latency or even deadlocks.
Even if it happens to work for Linux guests it seems like a strong
assumption that an rt-guest that has noise cores can deal with even more
noise one scheduling level below.
More recent proposals  suggest a scheme where the emulator and io
threads are on a separate core. That sounds more reasonable /
conservative but dramatically increases the per VM cost. And the pcpus
hosting the hypervisor threads will probably be idle most of the time.
I guess in this context the most important question is whether qemu is
ever involved in "regular operation" if you avoid the obvious IO
problems on your critical path.
My guess is that just  has serious hidden latency problems and 
is taking it a step too far by wasting whole cores for idle emulators.
We would like to suggest some other way inbetween, that is a little
easier on the core count. Our current solution seems to work fine but
has the mentioned quota problems.
With this mail i am hoping to collect some constraints to derive a
suggestion from. Or maybe collect some information that could be added
to the current blueprints as reasoning/documentation.
Sorry if you receive this mail a second time, i was not subscribed to
openstack-dev the first time.
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