Iris uses an Airspeed Velocity (ASV) setup to benchmark performance. This is primarily designed to check for performance shifts between commits using statistical analysis, but can also be easily repurposed for manual comparative and scalability analyses.

The benchmarks are automatically run overnight by a GitHub Action, with any notable shifts in performance being flagged in a new GitHub issue.

asv ... commands must be run from this directory. You will need to have ASV installed, as well as Nox (see Benchmark environments).

Iris' noxfile includes a benchmarks session that provides conveniences for setting up before benchmarking, and can also replicate the automated overnight run locally. See the session docstring for detail.

• OVERRIDE_TEST_DATA_REPOSITORY - required - some benchmarks use iris-test-data content, and your local site.cfg is not available for benchmark scripts.
• DATA_GEN_PYTHON - required - path to a Python executable that can be used to generate benchmark test objects/files; see Data generation. The Nox session sets this automatically, but will defer to any value already set in the shell.
• BENCHMARK_DATA - optional - path to a directory for benchmark synthetic test data, which the benchmark scripts will create if it doesn't already exist. Defaults to <root>/benchmarks/.data/ if not set. Note that some of the generated files, especially in the 'SPerf' suite, are many GB in size so plan accordingly.
• ON_DEMAND_BENCHMARKS - optional - when set (to any value): benchmarks decorated with @on_demand_benchmark are included in the ASV run. Usually coupled with the ASV --bench argument to only run the benchmark(s) of interest. Is set during the Nox cperf and sperf sessions.

See the ASV docs for full detail.

Important: be sure not to use the benchmarking environment to generate any test objects/files, as this environment changes with each commit being benchmarked, creating inconsistent benchmark 'conditions'. The generate_data module offers a solution; read more detail there.

Note that ASV re-runs a benchmark multiple times between its setup() routine. This is a problem for benchmarking certain Iris operations such as data realisation, since the data will no longer be lazy after the first run. Consider writing extra steps to restore objects' original state within the benchmark itself.

If adding steps to the benchmark will skew the result too much then re-running can be disabled by setting an attribute on the benchmark: number = 1. To maintain result accuracy this should be accompanied by increasing the number of repeats between setup() calls using the repeat attribute. warmup_time = 0 is also advisable since ASV performs independent re-runs to estimate run-time, and these will still be subject to the original problem.

When comparing performance between commits/file-type/whatever it can be helpful to know if the differences exist in scaling or non-scaling parts of the Iris functionality in question. This can be done using a size parameter, setting one value to be as small as possible (e.g. a scalar Cube), and the other to be significantly larger (e.g. a 1000x1000 Cube). Performance differences might only be seen for the larger value, or the smaller, or both, getting you closer to the root cause.

Some benchmarks provide useful insight but are inappropriate to be included in a benchmark run by default, e.g. those with long run-times or requiring a local file. These benchmarks should be decorated with @on_demand_benchmark (see benchmarks init), which sets the benchmark to only be included in a run when the ON_DEMAND_BENCHMARKS environment variable is set. Examples include the CPerf and SPerf benchmark suites for the UK Met Office NG-VAT project.

We have disabled ASV's standard environment management, instead using an environment built using the same Nox scripts as Iris' test environments. This is done using ASV's plugin architecture - see asv_delegated_conda.py and the extra config items in asv.conf.json.

(ASV is written to control the environment(s) that benchmarks are run in - minimising external factors and also allowing it to compare between a matrix of dependencies (each in a separate environment). We have chosen to sacrifice these features in favour of testing each commit with its intended dependencies, controlled by Nox + lock-files).