General purpose decision engine threadpool¶
https://blueprints.launchpad.net/watcher/+spec/general-purpose-decision-engine-threadpool
Many I/O operations such as writing to disk or waiting for network responses take a long amount of time. By leveraging parallelism the time taken to perform such operations can be drastically reduced. The general purpose threadpool for the decision engine will allow to perform methods in parallel, thereby reducing the time required to perform these operations.
Problem description¶
The general purpose threadpool will reduce the time it takes to perform certain operations of the decision engine. Currently, the gathering of metrics or the building of the data model can take large amounts of time proportionally for example.
Use Cases¶
As a user I want the execution of an audit to be performed as quickly as possible.
Proposed change¶
Introduce a general purpose threadpool using the futurist library for which end users can configure the amount of threads. This will introduce a new class that uses a singular pattern so it can be used throughout the decision engine. The threadpool will use the GreenThreadPoolExecutor as it does not conflict with other threadpools used in the decision engine. The singleton will contain several methods to simplify the submission of tasks and waiting for their completion.
With the threadpool the building of the data model will be parallelized which is done in three steps. First the calls to retrieve information about aggregates and availability zones are executed in parallel. The information is used to submit tasks to gather the information about each compute node. Finally, using futures will allow to immediately submit a task to retrieve the information about instances for a given compute node.
Alternatives¶
A purpose build threadpool specific for building the data model. However, this would mean that introducing parallelism in other parts of the decision engine would again require the design and implementation of yet another threadpool.
Data model impact¶
None
REST API impact¶
None
Security impact¶
None
Notifications impact¶
None
Other end user impact¶
None
Performance Impact¶
Overall the time take to perform audits should significantly decrease, however, a large amount of threads could negatively impact external services. Because of this the default amount of threads should be a relatively safe value for most OpenStack infrastructures.
Other deployer impact¶
Deployers should evaluate the amount of threads they want Watcher to use based on the scale of their infrastructure.
Developer impact¶
None
Implementation¶
Assignee(s)¶
- Primary assignee:
<dantalion>
Work Items¶
Implement the threadpool singleton based on previous proof of concepts.
Modify the method to build the data model to use the three step parallelism as described.
Implement relevant test cases to test the threadpool and the building of the data model.
Dependencies¶
futurist, A library maintained and developed by OpenStack.
Testing¶
Both the behavior of the building of the data model as well as the threadpool should be evaluated.
The verifying of behavior for helper methods in the threadpool.
The verifying of the completeness of the data model when build with parallelization.
Documentation Impact¶
The documentation will be updated to include usage examples that indicate to other developers how to best use the threadpool. Additionally, typical use cases in what situations this threadpool will be most useful will be provided.
References¶
History¶
Release Name |
Description |
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Ussuri |
Introduced |
