Enhance benchmark runner args and defaults (#5797)

There are some very useful default arguments, so teach the runner script
to directly provide them. They can be easily overridden if needed. As
part of this, change the default run count to 10 which much more often
produces statistically significant error bars and results.

Also, tweak the processing of the results to provide a stable order
based on the source order, even when randomized interleaving is enabled.
The randomized interleaving improves the statistical strength of the
benchmarks significantly, but displaying the results in the source order
is much more understandable. This should give roughly the best of both
worlds.

scripts/bench_runner.py

@@ -162,7 +162,7 @@ binary.
     )
     parser.add_argument(
         "--runs",
-        default=5,
+        default=10,
         metavar="N",
         type=int,
         help="Number of runs of the benchmark",
@@ -205,6 +205,19 @@ results, and just dumps the aggregate JSON data from the repeated runs.
     return parser.parse_args(args=args)
 
 
+# Default arguments that will be passed even when arguments are passed with
+# `--benchmark_args` to the script. These can be undone by overriding them in
+# explicitly passed arguments.
+DEFAULT_BENCHMARK_ARGS = [
+    # Randomize the order in which the benchmarks run to avoid skewed results
+    # due to a specific order.
+    "--benchmark_enable_random_interleaving",
+    # Reduce the default minimum time to 0.1s as it's more effective to use
+    # multiple runs to improve confidence in measurements.
+    "--benchmark_min_time=0.1s",
+]
+
+
 # Pre-compiled regexes to match metrics that measure _speed_: larger is better.
 SPEED_METRIC_PATTERNS = [
     re.compile(p)
@@ -688,12 +701,12 @@ def run_benchmark_binary(
     if len(binary_path.parts) == 1 and binary_path.exists():
         binary_str = f"./{binary_str}"
     run_cmd = (
-        [
-            binary_str,
-            "--benchmark_format=json",
-        ]
+        [binary_str]
+        + DEFAULT_BENCHMARK_ARGS
         + common_args
         + specific_args
+        # Pass the format flag last as it is required and can't be overridden.
+        + ["--benchmark_format=json"]
     )
     console.log(f"Executing: {' '.join(run_cmd)}")
 
@@ -761,6 +774,7 @@ def print_run_context(
 
 
 def get_benchmark_names_and_metrics(
+    console: Console,
     parsed_args: argparse.Namespace,
     exp_runs: list[dict],
     base_runs: list[dict],
@@ -779,10 +793,8 @@ def get_benchmark_names_and_metrics(
         - The list of unique benchmark names, maintaining their order.
         - The list of metrics to display.
     """
-    metrics: list[str] = []
-    benchmark_names: list[str] = []
-
     # Start with the base time and iteration metrics requested.
+    metrics: list[str] = []
     if parsed_args.wall_time:
         metrics.append("real_time")
     else:
@@ -803,12 +815,24 @@ def get_benchmark_names_and_metrics(
     if parsed_args.base_benchmark:
         one_run_benchmarks += base_runs[0]["benchmarks"]
 
+    benchmark_name_set: set[str] = set()
+    benchmark_name_indices: dict[str, tuple[int, int]] = {}
     for benchmark in one_run_benchmarks:
         name = benchmark["name"]
-        # Add the benchmark name if we haven't seen it before to get a unique
-        # list that preserves the order of appearance.
-        if name not in benchmark_names:
-            benchmark_names.append(name)
+        benchmark_name_set.add(name)
+        indices = (
+            benchmark["family_index"],
+            benchmark["per_family_instance_index"],
+        )
+        if name not in benchmark_name_indices:
+            benchmark_name_indices[name] = indices
+        else:
+            if benchmark_name_indices[name] != indices:
+                console.print(
+                    f"ERROR: Inconsintent indices {indices} and "
+                    f"{benchmark_name_indices[name]} for benchmark `{name}`."
+                )
+                sys.exit(1)
 
         # Add any extra metrics from this benchmark.
         for key in benchmark.keys():
@@ -818,6 +842,9 @@ def get_benchmark_names_and_metrics(
                 continue
             metrics.append(key)
 
+    benchmark_names = sorted(
+        list(benchmark_name_set), key=lambda name: benchmark_name_indices[name]
+    )
     return benchmark_names, metrics
 
 
@@ -1074,7 +1101,7 @@ def main() -> None:
 
     # Collect the benchmark names and metric names.
     benchmark_names, metrics = get_benchmark_names_and_metrics(
-        parsed_args, exp_runs, base_runs
+        console, parsed_args, exp_runs, base_runs
     )
 
     # Build any mappings between main benchmark names and comparables, and reset