# HG changeset patch
# User Antoine Pitrou <antoine@p.i.t.r.o.u.net>
# Date 1229556899 -3600

diff -r 99507aabe77a -r 5989f5396f02 Modules/gcmodule.c
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -63,6 +63,22 @@ static PyObject *gc_str = NULL;
 /* Python string used to look for __del__ attribute. */
 static PyObject *delstr = NULL;
 
+/* This is the number of objects who survived the last full collection. It
+   approximates the number of long lived objects tracked by the GC. */
+static Py_ssize_t long_lived_total = 0;
+/* This is the number of objects who survived the last "before-full"
+   collection, and awaiting to undergo a full collection for the first time.
+   
+   When there are many long-lived objects, the ratio
+	long_lived_pending / long_lived_total
+   is used to decide when to trigger a full collection
+   (in collect_generations()). Before Python 2.7, not doing so could lead
+   to quadratic performance degradation when keeping lots (e.g. millions)
+   of GC-enabled objects in memory.
+
+*/
+static Py_ssize_t long_lived_pending = 0;
+
 /* set for debugging information */
 #define DEBUG_STATS		(1<<0) /* print collection statistics */
 #define DEBUG_COLLECTABLE	(1<<1) /* print collectable objects */
@@ -817,8 +833,16 @@ collect(int generation)
 	move_unreachable(young, &unreachable);
 
 	/* Move reachable objects to next generation. */
-	if (young != old)
+	if (young != old) {
+		if (generation == NUM_GENERATIONS - 2) {
+			long_lived_pending += gc_list_size(young);
+		}
 		gc_list_merge(young, old);
+	}
+	else {
+		long_lived_pending = 0;
+		long_lived_total = gc_list_size(young);
+	}
 
 	/* All objects in unreachable are trash, but objects reachable from
 	 * finalizers can't safely be deleted.  Python programmers should take
@@ -918,6 +942,12 @@ collect_generations(void)
 	 * generations younger than it will be collected. */
 	for (i = NUM_GENERATIONS-1; i >= 0; i--) {
 		if (generations[i].count > generations[i].threshold) {
+			/* Avoid quadratic performance degradation in number
+			   of tracked objects. See issue #4074.
+			*/
+			if (i == NUM_GENERATIONS - 1
+			    && long_lived_pending < long_lived_total / 4)
+				continue;
 			n = collect(i);
 			break;
 		}