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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

OK so maybe this is just noise and if it's not causing any real issues
we could invalidate this bug.

** Changed in: nova
       Status: Confirmed => Invalid

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https://bugs.launchpad.net/bugs/1831758

Title:
  passthrough_whitelist is parsed per compute node per scheduling
  request even though the whitelist doesn't change between requests

Status in OpenStack Compute (nova):
  Invalid

Bug description:
  During each scheduling request we build a HostState object per compute
  node that is a candidate for scheduling:

  https://github.com/openstack/nova/blob/9a7eee1592d84c732415da639c85ce4b262eefab/nova/scheduler/host_manager.py#L796

  That eventually calls to _update_from_compute_node:

  Which creates a fresh PciDeviceStats object:

  https://github.com/openstack/nova/blob/9a7eee1592d84c732415da639c85ce4b262eefab/nova/scheduler/host_manager.py#L227

  And that object will parse the [pci]/passthrough_whitelist config
  every time it creates the PciDeviceStats object:

  https://github.com/openstack/nova/blob/9a7eee1592d84c732415da639c85ce4b262eefab/nova/pci/stats.py#L66

  That config option is not mutable so it's static per scheduling
  request on the same host until the nova-scheduler service is
  restarted.

  In a deployment that could return hundreds of allocation candidate
  compute nodes (max_placement_results defaults to 1000) we could spend
  a measurable amount of time parsing that same config option in
  aggregate when the resulting value won't change.

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