Run the json_view comparison on GitHub runners on demand

A workflow runs compare.py with pinned downloads on GitHub-hosted
Ubuntu runners and shows the results as the job summary and as an
artifact: started by hand (workflow_dispatch: x86-64 or AArch64, GCC or
Clang), or when a pull request gets the label "benchmark" (both
architectures, GCC). The label trigger gives numbers before the
workflow is on the default branch, which workflow_dispatch needs.
Shared runners are noisy, so the numbers show where json_view stands on
another architecture; published numbers still need a quiet machine.

compare.py takes the CPU name from lscpu where /proc/cpuinfo has none
(AArch64 Linux), and falls back to the architecture. Checked in Linux
containers (AArch64, Clang 15 and GCC 9, offline with the pinned
archives).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/benchmarks/json_view/README.md

@@ -29,6 +29,14 @@ python3 tests/benchmarks/json_view/compare.py --data <json_test_data directory>
 For numbers worth publishing, use a quiet machine (see [Getting stable numbers](../README.md#getting-stable-numbers)),
 the default 30 rounds or more, and `--native` only if the other libraries were built for the same CPU.
 
+### On GitHub-hosted runners
+
+The workflow [json_view benchmarks](../../../.github/workflows/json_view_benchmarks.yml) runs `compare.py --download`
+on demand: by hand (Actions → "json_view benchmarks" → "Run workflow"), on an x86-64 or AArch64 Ubuntu runner with GCC
+or Clang, or when a pull request gets the label `benchmark`, on both architectures with GCC. The results appear as the
+job summary and as an artifact. Shared runners are noisy, so these numbers show
+where `json_view` stands on another architecture; they are not meant for publication.
+
 ## What is measured
 
 `bench_view.cpp` runs four workloads on twitter, citm_catalog, canada, jeopardy, a single tweet (`status`), and a

tests/benchmarks/json_view/compare.py

@@ -183,7 +183,11 @@ def cpu_model():
                     return line.split(':', 1)[1].strip()
     except OSError:
         pass
-    return platform.processor()
+    # (AArch64 Linux: /proc/cpuinfo has no model name, lscpu knows it)
+    for line in output(['lscpu']).splitlines():
+        if line.startswith('Model name:'):
+            return line.split(':', 1)[1].strip()
+    return platform.processor() or platform.machine()
 
 
 def git_commit():