phsm opened a new pull request, #10450:
URL: https://github.com/apache/cloudstack/pull/10450
### Description
This PR speeds up Prometheus exporter reply generation by ensuring the same
host tag is not polled multiple times.
<!--- Describe your changes in DETAIL - And how has behaviour functionally
changed. -->
It utilizes `HashSet` instead of `ArrayList` to store the host tags,
therefore deduplicates the tags when they are present on multiple hosts.
It fixes two bugs:
1. Prevents duplication of the `cloudstack_vms_total_by_tag{filter="<vm
state>", zone="<zonename>", tags="<tagname>"}` in the reply.
2. Speeds up forming Prometheus exporter reply on large Cloudstack
installations when multiple hosts with repeating tags are used, for example 200
hosts but only 5 unique host tags. The time to get the Prometheus exporter
reply has reduced from 4.5 mins to 26 seconds in my setup.
<!-- For new features, provide link to FS, dev ML discussion etc. -->
<!-- In case of bug fix, the expected and actual behaviours, steps to
reproduce. -->
## Steps to reproduce
1. Take a large Cloudstack installation with > 100 hosts
2. Populate these hosts with the several unique tags (e.g. 5 tags). So the
host1 gets some of those tags, host2 gets the same tags, host3 gets the same
tags etc.
4. `time curl http://<cs.server.ip>:<prometheus.port>/metrics` from the
prometheus exporter port. Inspect the reply, and note the time it took to
finish.
5. Apply this patch.
6. Run the same command from the Step 3, and check the difference in time
taken to process the request.
Expected behavior:
- The metrics with the same name + labels, such as
`cloudstack_vms_total_by_tag{filter="<vm state>", zone="<zonename>",
tags="<tagname>"}` shall not be duplicated in the reply.
- The processing time shall not be taking more than a minute or so.
Actual behavior:
- the `cloudstack_vms_total_by_tag` metrics with the same labels are not
unique
- it takes a longer time to process, than it is necessary
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closed when this PR gets merged -->
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### Types of changes
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] Enhancement (improves an existing feature and functionality)
- [ ] Cleanup (Code refactoring and cleanup, that may add test cases)
- [ ] build/CI
- [ ] test (unit or integration test code)
### Feature/Enhancement Scale or Bug Severity
#### Feature/Enhancement Scale
- [ ] Major
- [ ] Minor
#### Bug Severity
- [ ] BLOCKER
- [ ] Critical
- [ ] Major
- [ ] Minor
- [X] Trivial
### Screenshots (if appropriate):
### How Has This Been Tested?
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#### How did you try to break this feature and the system with this change?
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