# Testing Dawn

(TODO)

## Dawn Perf Tests

For benchmarking with `dawn_perf_tests`, it's best to build inside a Chromium checkout using the following GN args:
```
is_official_build = true  # Enables highest optimization level, using LTO on some platforms
use_dawn = true           # Required to build Dawn
use_cfi_icall=false       # Required because Dawn dynamically loads function pointers, and we don't sanitize them yet.
```

A Chromium checkout is required for the highest optimization flags. It is possible to build and run `dawn_perf_tests` from a standalone Dawn checkout as well, only using GN arg `is_debug=false`. For more information on building, please see [building.md](./building.md).

### Terminology

 - Iteration: The unit of work being measured. It could be a frame, a draw call, a data upload, a computation, etc. `dawn_perf_tests` metrics are reported as time per iteration.
 - Step: A group of Iterations run together. The number of `iterationsPerStep` is provided to the constructor of `DawnPerfTestBase`.
 - Trial: A group of Steps run consecutively. `kNumTrials` are run for each test. A Step in a Trial is run repetitively for approximately `kCalibrationRunTimeSeconds`. Metrics are accumlated per-trial and reported as the total time divided by `numSteps * iterationsPerStep`. `maxStepsInFlight` is passed to the `DawnPerfTestsBase` constructor to limit the number of Steps pipelined.

(See [`//src/tests/perf_tests/DawnPerfTest.h`](https://cs.chromium.org/chromium/src/third_party/dawn/src/tests/perf_tests/DawnPerfTest.h) for the values of the constants).

### Metrics

`dawn_perf_tests` measures the following metrics:
 - `wall_time`: The time per iteration, including time waiting for the GPU between Steps in a Trial.
 - `cpu_time`: The time per iteration, not including time waiting for the GPU between Steps in a Trial.
 - `validation_time`: The time for CommandBuffer / RenderBundle validation.
 - `recording_time`: The time to convert Dawn commands to native commands.

Metrics are reported according to the format specified at
[[chromium]//build/scripts/slave/performance_log_processor.py](https://cs.chromium.org/chromium/build/scripts/slave/performance_log_processor.py)

### Dumping Trace Files

The test harness supports a `--trace-file=path/to/trace.json` argument where Dawn trace events can be dumped. The traces can be viewed in Chrome's `about://tracing` viewer.

### Test Runner

[`//scripts/perf_test_runner.py`](https://cs.chromium.org/chromium/src/third_party/dawn/scripts/perf_test_runner.py) may be run to continuously run a test and report mean times and variances.

Currently the script looks in the `out/Release` build directory and measures the `wall_time` metric (hardcoded into the script). These should eventually become arguments.

Example usage:

```
scripts/perf_test_runner.py DrawCallPerf.Run/Vulkan__e_skip_validation
```

### Tests

**BufferUploadPerf**

Tests repetitively uploading data to the GPU using either `WriteBuffer` or `CreateBuffer` with `mappedAtCreation = true`.

**DrawCallPerf**

DrawCallPerf tests drawing a simple triangle with many ways of encoding commands,
binding, and uploading data to the GPU. The rationale for this is the following:
  - Static/Multiple/Dynamic vertex buffers: Tests switching buffer bindings. This has
    a state tracking cost as well as a GPU driver cost.
  - Static/Multiple/Dynamic bind groups: Same rationale as vertex buffers
  - Static/Dynamic pipelines: In addition to a change to GPU state, changing the pipeline
    layout incurs additional state tracking costs in Dawn.
  - With/Without render bundles: All of the above can have lower validation costs if
    precomputed in a render bundle.
  - Static/Dynamic data: Updating data for each draw is a common use case. It also tests
    the efficiency of resource transitions.