Add the PYTHON_GC_THRESHOLD environment variable.

Author: nascheme

Doc/using/cmdline.rst

@@ -976,6 +976,16 @@ conflict.
    .. versionadded:: 3.4
 
 
+.. envvar:: PYTHON_GC_THRESHOLD
+
+   Set the `threshold0` value for the garbage collector.  This is the same
+   as calling `gc.set_threshold(n)` where `n` is value of the variable.  Using
+   an empty value or the value "default" will cause the default threshold to be
+   used.
+
+   .. versionadded:: 3.13
+
+
 .. envvar:: PYTHONMALLOC
 
    Set the Python memory allocators and/or install debug hooks.

Misc/NEWS.d/next/Core_and_Builtins/2024-09-28-20-01-40.gh-issue-124742.7kJx88.rst

@@ -0,0 +1,4 @@
+Add the `PYTHON_GC_THRESHOLD` environment variable.  This can be used to set
+the `threshold0` value for the garbage collector, similar to calling
+`gc.set_threshold()`.  If the value is empty or "default", the default
+threshold value is used.

Python/gc.c

@@ -166,12 +166,31 @@ _PyGC_InitState(GCState *gcstate)
 #undef INIT_HEAD
 }
 
+static void
+gc_set_threshold_from_env(PyInterpreterState *interp)
+{
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+    const char *env = _Py_GetEnv(config->use_environment,
+                                 "PYTHON_GC_THRESHOLD");
+    if (env == NULL || strcmp(env, "default") == 0) {
+        return;
+    }
+    int threshold = -1;
+    if (_Py_str_to_int(env, &threshold) < 0) {
+        return; // parse failed, silently ignore
+    }
+    if (threshold > 0) {
+        interp->gc.young.threshold = threshold;
+    }
+}
 
 PyStatus
 _PyGC_Init(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
 
+    gc_set_threshold_from_env(interp);
+
     gcstate->garbage = PyList_New(0);
     if (gcstate->garbage == NULL) {
         return _PyStatus_NO_MEMORY();

Python/gc_free_threading.c

@@ -838,12 +838,31 @@ _PyGC_InitState(GCState *gcstate)
     gcstate->young.threshold = 2000;
 }
 
+static void
+gc_set_threshold_from_env(PyInterpreterState *interp)
+{
+    const PyConfig *config = _PyInterpreterState_GetConfig(interp);
+    const char *env = _Py_GetEnv(config->use_environment,
+                                 "PYTHON_GC_THRESHOLD");
+    if (env == NULL || strcmp(env, "default") == 0) {
+        return;
+    }
+    int threshold = -1;
+    if (_Py_str_to_int(env, &threshold) < 0) {
+        return; // parse failed, silently ignore
+    }
+    if (threshold > 0) {
+        interp->gc.young.threshold = threshold;
+    }
+}
 
 PyStatus
 _PyGC_Init(PyInterpreterState *interp)
 {
     GCState *gcstate = &interp->gc;
 
+    gc_set_threshold_from_env(interp);
+
     gcstate->garbage = PyList_New(0);
     if (gcstate->garbage == NULL) {
         return _PyStatus_NO_MEMORY();

Python/initconfig.c

@@ -342,6 +342,8 @@ static const char usage_envvars[] =
 "                  on Python memory allocators.  Use PYTHONMALLOC=debug to\n"
 "                  install debug hooks.\n"
 "PYTHONMALLOCSTATS: print memory allocator statistics\n"
+"PYTHON_GC_THRESHOLD: set threshold0 for the garbage collector.  This\n"
+"                  threshold can also be set by gc.set_threshold().\n"
 "PYTHONCOERCECLOCALE: if this variable is set to 0, it disables the locale\n"
 "                  coercion behavior.  Use PYTHONCOERCECLOCALE=warn to request\n"
 "                  display of locale coercion and locale compatibility warnings\n"