160e52ec7e
Resolves #2054 **Context**: branch creation needs to wait for GC to acquire `gc_cs` lock, which prevents creating new timelines during GC. However, because individual timeline GC iteration also requires `compaction_cs` lock, branch creation may also need to wait for compactions of multiple timelines. This results in large latency when creating a new branch, which we advertised as *"instantly"*. This PR optimizes the latency of branch creation by separating GC into two phases: 1. Collect GC data (branching points, cutoff LSNs, etc) 2. Perform GC for each timeline The GC bottleneck comes from step 2, which must wait for compaction of multiple timelines. This PR modifies the branch creation and GC functions to allow GC to hold the GC lock only in step 1. As a result, branch creation doesn't need to wait for compaction to finish but only needs to wait for GC data collection step, which is fast.
What performance tests do we have and how we run them
Performance tests are built using the same infrastructure as our usual python integration tests. There are some extra fixtures that help to collect performance metrics, and to run tests against both vanilla PostgreSQL and Neon for comparison.
Tests that are run against local installation
Most of the performance tests run against a local installation. This is not very representative of a production environment. Firstly, Postgres, safekeeper(s) and the pageserver have to share CPU and I/O resources, which can add noise to the results. Secondly, network overhead is eliminated.
In the CI, the performance tests are run in the same environment as the other integration tests. We don't have control over the host that the CI runs on, so the environment may vary widely from one run to another, which makes the results across different runs noisy to compare.
Remote tests
There are a few tests that marked with pytest.mark.remote_cluster. These tests do not set up a local environment, and instead require a libpq connection string to connect to. So they can be run on any Postgres compatible database. Currently, the CI runs these tests our staging environment daily. Staging is not an isolated environment, so there can be noise in the results due to activity of other clusters.
Noise
All tests run only once. Usually to obtain more consistent performance numbers, a test should be repeated multiple times and the results be aggregated, for example by taking min, max, avg, or median.
Results collection
Local test results for main branch, and results of daily performance tests, are stored in a neon project deployed in production environment. There is a Grafana dashboard that visualizes the results. Here is the dashboard. The main problem with it is the unavailability to point at particular commit, though the data for that is available in the database. Needs some tweaking from someone who knows Grafana tricks.
There is also an inconsistency in test naming. Test name should be the same across platforms, and results can be differentiated by the platform field. But currently, platform is sometimes included in test name because of the way how parametrization works in pytest. I.e. there is a platform switch in the dashboard with zenith-local-ci and zenith-staging variants. I.e. some tests under zenith-local-ci value for a platform switch are displayed as Test test_runner/performance/test_bulk_insert.py::test_bulk_insert[vanilla] and Test test_runner/performance/test_bulk_insert.py::test_bulk_insert[zenith] which is highly confusing.