PEP 848: Clarifications and bug fixes

peps/pep-0848.rst

@@ -201,16 +201,14 @@ When the nursery is full, the unreachable cycles in the oldest aging space
            reachable to visited. They are reachable and cannot be garbage.
         """
         moved_to_visited = 0
-        while reachable:
+        while reachable and moved_to_visited < limit:
             root = reachable.pop()
             visited.append(root)
             moved_to_visited += 1
             for obj in gc.get_referents(root):
                 if obj in pending:
                     pending.remove(obj)
                     reachable.append(obj)
-        if moved_to_visited >= limit:
-            return moved_to_visited
         return moved_to_visited
 
     def old_collection():
@@ -230,14 +228,14 @@ When the nursery is full, the unreachable cycles in the oldest aging space
             # form transitive closure starting at obj, taking objects from pending
             increment = form_transitive_closure(obj, pending)
             candidates = len(increment)
+            work_to_do -= candidates
             survivors = collect_cycles(increment)
-            work_to_do -= survivors
-            collected = candidates - survivors
+            collected = candidates - len(survivors)
+            visited_space.extend(survivors)
             # If we are collecting lots of objects, that means
-            # there is a lot of cycle garbage and we need to
-            # sweep the heap faster.
+            # there is a lot of cycle garbage and we should sweep
+            # the heap faster to keep the amount of garbage down
             work_to_do += 2 * collected
-            visited_space.extend(survivors)
 
 The legacy collector
 --------------------
@@ -283,11 +281,17 @@ of half spaces is always rounded up to an even number.
 Performance
 -----------
 
-Performance is improved relative to the current collector.
-The performance improvements come from doing less work in the young
-generations (one collection per object, not two) and doing less work
-in the old generation due to the lower survivor rate from the young
-generations.
+The new collector reduces the overhead of cyclic garbage collection by almost
+half, although the exact amount depends on the application.
+
+By allowing objects longer to die, the effectiveness of the collector is
+improved. This allows it to collect the same amount of garbage for less work.
+Performance is further improved by scanning fewer objects during collections:
+
+* In the young generation: each object is only scanned once, instead of twice
+  in the generational GC
+* In the old generation: objects are scanned at a lower rate, only increasing
+  that rate when necessary to collect excess garbage
 
 Peak Memory Consumption
 -----------------------
@@ -322,10 +326,10 @@ Calling ``gc.collect()`` is equivalent to calling ``gc.collect(2)``.
 ================== ==============================================
  Argument                   Effect
 ================== ==============================================
-    0               Perform a young collection,
-                    collecting the oldest aging space
-    1               Collect an increment of the old generation
- 2 or no argument   Collect the whole heap
+    0                Perform a young collection,
+                     collecting the oldest aging space
+    1                Collect an increment of the old generation
+ 2 or no argument    Collect the whole heap
 ================== ==============================================
 
 Choosing the legacy generational collector
@@ -398,24 +402,6 @@ collections will usually mean shorter pauses per collection.
 Future work
 ===========
 
-Further reducing pause times
-----------------------------
-
-While the reference implementation is 1-2% faster than main (with the
-generational GC), it can still have long pause times on large object graphs.
-Many of the benchmarks have a single large tree as their object graph, and this
-can result in long pauses, as an increment starting at the root of the tree
-will contain almost the whole heap.
-
-This could be improved in a few ways:
-
-* Sorting the increments as they are either created or sent to the old
-  generation, so that the objects farthest from the root are picked first in
-  the next collection.
-* Traversing the stack prior to increment formation to skip reachable objects.
-  This will complicate the algorithm, but could save significant amounts of
-  work in some cases.
-
 Porting to the free-threaded build
 ----------------------------------
 
@@ -431,6 +417,18 @@ Porting the incremental GC to the free-threaded build will need a few changes:
   external arrays for the young generation and the increments. The
   free-threaded GC already needs to do this to partition garbage and survivors.
 
+Further reducing pause times
+----------------------------
+
+While the reference implementation generally has shorter pause times, it
+can still have long pause times if the transitive closure needed for an
+increment is large.
+
+It may be possible to scan increments over multiple collections, keeping
+each pause short. This would be challenging as the program may transform the
+object graph of the increment between collections, but garbage cycles cannot be
+modified by the program so this might be possible. There is extensive research
+on concurrent collectors which also have to handle similar problems.
 
 Reference Implementation
 ========================