low-memory-usage.yaml should lower NovaReservedHostMemory to 512MB

[Andreas Karis]

Description of problem:
Since Queens (perhaps Pike?), nova-compute reserved host memory by default is 4096. This doesn't work out well in low memory environments:
~~~
(overcloud) [stack@undercloud-1 ~]$ grep 4096 -C10 /usr/share/openstack-tripleo-heat-templates/puppet/services/nova-compute.yaml
      processes.
      Ex. NovaVcpuPinSet: ['4-12','^8'] will reserve cores from 4-12 excluding 8
    type: comma_delimited_list
    default: []
    tags:
      - role_specific
  NovaReservedHostMemory:
    description: >
      Reserved RAM for host processes.
    type: number
    default: 4096
    constraints:
      - range: { min: 512 }
    tags:
      - role_specific
  MonitoringSubscriptionNovaCompute:
    default: 'overcloud-nova-compute'
    type: string
  NovaComputeLoggingSource:
    type: json
    default:
~~~

~~~
[root@overcloud-compute-0 ~]# grep reserv /var/lib/config-data/puppet-generated/nova_libvirt -R | egrep -v ':#'
/var/lib/config-data/puppet-generated/nova_libvirt/etc/nova/nova.conf:reserved_host_memory_mb=4096
[root@overcloud-compute-0 ~]#
~~~

~~~
(overcloud) [stack@undercloud-1 ~]$ nova hypervisor-show d787b5c3-d83f-4246-9338-5083bfbb6058 | grep mb
| free_ram_mb | -1 |
| memory_mb | 4095 |
| memory_mb_used | 4096 |
(overcloud) [stack@undercloud-1 ~]$
~~~

The templates provide a low-memory-usage.yaml file:
/usr/share/openstack-tripleo-heat-templates/environments/low-memory-usage.yaml

Change that .yaml to include NovaReservedHostMemory: 512
~~~
# Lower the memory usage of overcloud.
parameter_defaults:
  CinderWorkers: 1
  GlanceWorkers: 1
  HeatWorkers: 1
  KeystoneWorkers: 1
  NeutronWorkers: 1
  NovaWorkers: 1
  SaharaWorkers: 1
  SwiftWorkers: 1
  GnocchiMetricdWorkers: 1

  ApacheMaxRequestWorkers: 100
  ApacheServerLimit: 100

  ControllerExtraConfig:
      'nova::network::neutron::neutron_url_timeout': '60'

  DatabaseSyncTimeout: 900

  # Override defaults to get HEALTH_OK with 1 OSD (for testing only)
  CephPoolDefaultSize: 1
  CephPoolDefaultPgNum: 32

  NovaReservedHostMemory: 512
~~~

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/577938

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/577938
Committed: https://git.openstack.org/cgit/openstack/tripleo-heat-templates/commit/?id=2022f09de57a933a208b4195feab096c81cc3df0
Submitter: Zuul
Branch: master

commit 2022f09de57a933a208b4195feab096c81cc3df0
Author: Andreas Karis <email address hidden>
Date: Mon Jun 25 18:02:59 2018 -0400

    Lower reserved memory for nova-compute

    Lower reserved memory for nova compute nodes in a
    memory restricted environment.

    Change-Id: I75635ab99d4d55628f6b32446beb14ae7333575a
    Closes-Bug: #1778597

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/queens
Review: https://review.openstack.org/581201

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/tripleo-heat-templates 9.0.0.0b4 development milestone.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/581201
Committed: https://git.openstack.org/cgit/openstack/tripleo-heat-templates/commit/?id=4765282b5c1d57d4bc110899bf1dbcf6e2095d3b
Submitter: Zuul
Branch: stable/queens

commit 4765282b5c1d57d4bc110899bf1dbcf6e2095d3b
Author: Andreas Karis <email address hidden>
Date: Mon Jun 25 18:02:59 2018 -0400

    Lower reserved memory for nova-compute

    Lower reserved memory for nova compute nodes in a
    memory restricted environment.

    Change-Id: I75635ab99d4d55628f6b32446beb14ae7333575a
    Closes-Bug: #1778597
    (cherry picked from commit 2022f09de57a933a208b4195feab096c81cc3df0)

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/tripleo-heat-templates 8.0.6 release.