Mark Goddard 2648f48746 Make setup module arguments configurable

Ansible facts can have a large impact on the performance of the Ansible
control host. This patch introduces some control over which facts are
gathered (kayobe_ansible_setup_gather_subset) and which facts are stored
(kayobe_ansible_setup_filter). By default we do not change the default
values of these arguments to the setup module. The flexibility of these
arguments is limited, but they do provide enough for a large performance
improvement in a typical moderate to large OpenStack cloud.

In particular, the large complex dict fact for each interface has a
large effect, and on an OpenStack controller or hypervisor there may be
many virtual interfaces. We can use the kayobe_ansible_setup_filter
variable to help:

    kayobe_ansible_setup_filter: 'ansible_[!qt]*'

This causes Ansible to collect but not store facts matching that
pattern, which includes the virtual interface facts. Currently we are
not referencing other facts matching the pattern within Kayobe.
Note that including the 'ansible_' prefix causes meta facts module_setup
and gather_subset to be filtered, but this seems to be the only way to
get a good match on the interface facts. To work around this, we use
ansible_facts rather than module_setup to detect whether facts exist in
the cache.

The exact improvement will vary, but has been reported to be as large as
18x on systems with many virtual interfaces.

This change also introduces a new command to gather facts for Kayobe &
Kolla Ansible on demand, 'kayobe overcloud facts gather'. This can be
used to populate a fact cache.

Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/794610
Story: 2007993
Task: 42586

Change-Id: I5ce3c734433e1682ee942867505468c57440e689

2021-09-27 09:31:09 +00:00

5.1 KiB

Raw Blame History

Ansible

Ansible configuration is described in detail in the Ansible documentation. It is explained elsewhere in this guide how to configure Ansible for Kayobe <configuration-kayobe-ansible> and Kolla Ansible <configuration-kolla-ansible-ansible>.

In this section we cover some options for tuning Ansible for performance and scale.

SSH pipelining

SSH pipelining is disabled in Ansible by default, but is generally safe to enable, and provides a reasonable performance improvement.

[ssh_connection]
pipelining = True

Forks

By default Ansible executes tasks using a fairly conservative 5 process forks. This limits the parallelism that allows Ansible to scale. Most Ansible control hosts will be able to handle far more forks than this. You will need to experiment to find out the CPU, memory and IO limits of your machine.

For example, to increase the number of forks to 20:

[defaults]
forks = 20

Fact caching

Note

Fact caching will not work correctly in Kayobe prior to the Ussuri release.

By default, Ansible gathers facts for each host at the beginning of every play, unless gather_facts is set to false. With a large number of hosts this can result in a significant amount of time spent gathering facts.

One way to improve this is through Ansible's support for fact caching. In order to make this work with Kayobe, it is necessary to change Ansible's gathering configuration option to smart. Additionally, it is necessary to use separate fact caches for Kayobe and Kolla Ansible due to some of the facts (e.g. ansible_facts.user_uid and ansible_facts.python) differing.

Example

In the following example we configure Kayobe and Kolla Ansible to use fact caching using the jsonfile cache plugin.

[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = /tmp/kayobe-facts

[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = /tmp/kolla-ansible-facts

You may also wish to set the expiration timeout for the cache via [defaults] fact_caching_timeout.

Fact gathering

Fact filtering

Filtering of facts can be used to speed up Ansible. Environments with many network interfaces on the network and compute nodes can experience very slow processing with Kayobe and Kolla Ansible. This happens due to the processing of the large per-interface facts with each task. To avoid storing certain facts, we can use the kayobe_ansible_setup_filter variable, which is used as the filter argument to the setup module.

One case where this is particularly useful is to avoid collecting facts for virtual tap (beginning with t) and bridge (beginning with q) interfaces created by Neutron. These facts are large map values which can consume a lot of resources on the Ansible control host. Kayobe and Kolla Ansible typically do not need to reference them, so they may be filtered. For example, to avoid collecting facts beginning with q or t:

kayobe_ansible_setup_filter: "ansible_[!qt]*"

Similarly, for Kolla Ansible (notice the similar but different file names):

kolla_ansible_setup_filter: "ansible_[!qt]*"

This causes Ansible to collect but not store facts matching that pattern, which includes the virtual interface facts. Currently we are not referencing other facts matching the pattern within Kolla Ansible. Note that including the 'ansible' prefix causes meta facts module_setup and gather_subset to be filtered, but this seems to be the only way to get a good match on the interface facts.

The exact improvement will vary, but has been reported to be as large as 18x on systems with many virtual interfaces.

Fact gathering subsets

It is also possible to configure which subsets of facts are gathered, via kayobe_ansible_setup_gather_subset, which is used as the gather_subset argument to the setup module. For example, if one wants to avoid collecting facts via facter:

kayobe_ansible_setup_gather_subset: "all,!facter"