[openstack-dev] [nova] NUMA + SR-IOV

Sergey Nikitin snikitin at mirantis.com
Thu Mar 24 16:18:35 UTC 2016

Hi, folks.

I want to start a discussion about NUMA + SR-IOV environment. I have a
two-sockets server. It has two NUMA nodes and only one SR-IOV PCI device.
This device is associated with the first NUMA node. I booted a set of VMs
with SR-IOV support. Each of these VMs was booted on the first NUMA node.
As I understand it happened for better performance (VM should be booted in
NUMA node which has PCI device for this VM) [1].

But this behavior leaves my 2-sockets machines half-populated. What if I
don't care about SR-IOV performance? I just want every VM from *any* of
NUMA nodes to use this single SR-IOV PCI device.

But I can't do it because of behavior of numa_topology_filter. In this
filter we want to know if current host has required PCI device [2]. But we
want to have this device *only* in some numa cell on this host. It is
hardcoded here [3]. If we do *not* pass variable "cells" to the method
support_requests() [4] we will boot VM on the current host, if it has
required PCI device *on host* (maybe not in the same NUMA node).

So my question is:
Is it correct that we *always* want to boot VM in NUMA node associated with
requested PCI device and user has no choice?
Or should we give a choice to the user and let him boot a VM with PCI
device, associated with another NUMA node?

[4] https://github.com/openstack/nova/blob/master/nova/pci/stats.py#L277
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