Use os.sched_getaffinity for parallelism

scripts/monitor_fuzz.py

@@ -31,6 +31,20 @@
 import time
 
 
+# Duplicated from test.shared (importing test.shared has unwanted side effects).
+def get_num_cores():
+    # Prefer `os.process_cpu_count` when available (3.13 and above) since it
+    # takes into account thread affinity. Fall back to `os.sched_getaffinity`
+    # where available and finally `os.cpu_count`, which should work everywhere.
+    if hasattr(os, 'process_cpu_count'):
+        cpu_count = os.process_cpu_count()
+    elif hasattr(os, 'sched_getaffinity'):
+        cpu_count = len(os.sched_getaffinity(0))
+    else:
+        cpu_count = os.cpu_count()
+    return int(os.getenv('BINARYEN_CORES', cpu_count))
+
+
 class FuzzMonitor:
     """Monitors fuzzer output stream, manages log files, and tracks state."""
 
@@ -164,7 +178,7 @@ def parse_args():
     binaryen_root = os.path.dirname(
         os.path.dirname(os.path.abspath(__file__)))
     default_log_dir = os.path.join(binaryen_root, 'out', 'test')
-    cores = os.cpu_count() or 1
+    cores = get_num_cores() or 1
     parser = argparse.ArgumentParser(description=__doc__)
     parser.add_argument(
         '-j',

scripts/test/shared.py

@@ -141,12 +141,26 @@ def red_stderr():
     return red_output(file=sys.stderr)
 
 
+# Adapted from Emscripten's tools/utils.py
+def get_num_cores():
+    # Prefer `os.process_cpu_count` when available (3.13 and above) since it
+    # takes into account thread affinity. Fall back to `os.sched_getaffinity`
+    # where available and finally `os.cpu_count`, which should work everywhere.
+    if hasattr(os, 'process_cpu_count'):
+        cpu_count = os.process_cpu_count()
+    elif hasattr(os, 'sched_getaffinity'):
+        cpu_count = len(os.sched_getaffinity(0))
+    else:
+        cpu_count = os.cpu_count()
+    return int(os.getenv('BINARYEN_CORES', cpu_count))
+
+
 def run_parallel_tests(run_one_test_func, tests, show_worker_count=True):
     global num_failures
     tests = list(tests)
     if not tests:
         return
-    worker_count = min(os.cpu_count() or 1, len(tests))
+    worker_count = min(get_num_cores() or 1, len(tests))
     if show_worker_count:
         print(f"Running with {worker_count} workers")