PEP 683: Updates for Round 4 of Discussions (gh-2766)

pep-0683.rst

@@ -72,14 +72,19 @@ Here's a high-level look at the implementation:
 If an object's refcount matches a very specific value (defined below)
 then that object is treated as immortal.  The CPython C-API and runtime
 will not modify the refcount (or other runtime state) of an immortal
-object.
+object.  The runtime will now be explicitly responsible for deallocating
+all immortal objects during finalization, unless statically allocated.
+(See `Object Cleanup`_ below.)
 
 Aside from the change to refcounting semantics, there is one other
 possible negative impact to consider.  The threshold for an "acceptable"
 performance penalty for immortal objects is 2% (the consensus at the
 2022 Language Summit).  A naive implementation of the approach described
-below makes CPython roughly 6% slower.  However, the implementation
-is performance-neutral once known mitigations are applied.
+below makes CPython roughly 4% slower.  However, the implementation
+is ~performance-neutral~ once known mitigations are applied.
+
+TODO: Update the performance impact for the latest branch
+(both for GCC and for clang).
 
 
 Motivation
@@ -95,7 +100,7 @@ a consistent high volume of refcount changes.
 
 The effective mutability of all Python objects has a concrete impact
 on parts of the Python community, e.g. projects that aim for
-scalability like Instragram or the effort to make the GIL
+scalability like Instagram or the effort to make the GIL
 per-interpreter.  Below we describe several ways in which refcount
 modification has a real negative effect on such projects.
 None of that would happen for objects that are truly immutable.
@@ -152,9 +157,10 @@ refcount semantics drastically reduce the benefits and
 have led to some sub-optimal workarounds.
 
 Also note that "fork" isn't the only operating system mechanism
-that uses copy-on-write semantics.  Anything that uses ``mmap``
-relies on copy-on-write, including sharing data from shared object
-files between processes.
+that uses copy-on-write semantics.  Another example is ``mmap``.
+Any such utility will potentially benefit from fewer copy-on-writes
+when immortal objects are involved, when compared to using only
+"mortal" objects.
 
 
 Rationale
@@ -198,24 +204,29 @@ immortal objects greatly simplifies the solution for existing static
 types, as well as objects exposed by the public C-API.
 
 In general, a strong immutability guarantee for objects enables Python
-applications to scale like never before.  This is because they can
-then leverage multi-core parallelism without a tradeoff in memory
-usage.  This is reflected in most of the above cases.
+applications to scale better, particularly in
+`multi-process deployments <Facebook>`_.  This is because they can then
+leverage multi-core parallelism without such a significant tradeoff in
+memory usage as they now have.  The cases we just described, as well as
+those described above in `Motivation`_, reflect this improvement.
 
 Performance
 -----------
 
 A naive implementation shows `a 4% slowdown`_.  We have demonstrated
-a return to performance-neutral with a handful of basic mitigations
+a return to ~performance-neutral~ with a handful of basic mitigations
 applied.  See the `mitigations`_ section below.
 
 On the positive side, immortal objects save a significant amount of
-memory when used with a pre-fork model.  Also, immortal objects provide
-opportunities for specialization in the eval loop that would improve
-performance.
+memory when used `with a pre-fork model <Facebook>`_.  Also, immortal
+objects provide opportunities for specialization in the eval loop that
+would improve performance.
 
 .. _a 4% slowdown: https://github.com/python/cpython/pull/19474#issuecomment-1032944709
 
+TODO: Update the performance impact for the latest branch
+(both for GCC and for clang).
+
 Backward Compatibility
 ----------------------
 
@@ -503,9 +514,10 @@ mutable object won't actually be modified.
 On the other hand, some mutable objects will never be shared between
 threads (at least not without a lock like the GIL).  In some cases it
 may be practical to make some of those immortal too.  For example,
-``sys.modules`` is a per-interpreter dict that we do not expect to ever
-get freed until the corresponding interpreter is finalized.  By making
-it immortal, we no longer incur the extra overhead during incref/decref.
+``sys.modules`` is a per-interpreter dict that we do not expect to
+ever get freed until the corresponding interpreter is finalized
+(assuming it isn't replaced).  By making it immortal, we would
+no longer incur the extra overhead during incref/decref.
 
 We explore this idea further in the `mitigations`_ section below.
 
@@ -520,7 +532,8 @@ it.
 Examples:
 
 * containers like ``dict`` and ``list``
-* objects that hold references internally like ``PyTypeObject.tp_subclasses``
+* objects that hold references internally like ``PyTypeObject`` with
+  its ``tp_subclasses`` and ``tp_weaklist``
 * an object's type (held in ``ob_type``)
 
 Such held objects are thus implicitly immortal for as long as they are
@@ -573,6 +586,9 @@ stable ABI extension).
 Note that top two bits of the refcount are already reserved for other
 uses.  That's why we are using the third top-most bit.
 
+The implementation is also open to using other values for the immortal
+bit, such as the sign bit or 2^31 (for saturated refcounts on 64-bit).
+
 Affected API
 ------------
 
@@ -588,9 +604,11 @@ API that exposes refcounts (unchanged but may now return large values):
 * (public) ``Py_REFCNT()``
 * (public) ``sys.getrefcount()``
 
-(Note that ``_Py_RefTotal`` and ``sys.gettotalrefcount()``
+(Note that ``_Py_RefTotal``, and consequently ``sys.gettotalrefcount()``,
 will not be affected.)
 
+TODO: clarify the status of ``_Py_RefTotal``.
+
 Also, immortal objects will not participate in GC.
 
 Immortal Global Objects
@@ -678,6 +696,7 @@ other possibilities
 * mark the "interned" dict as immortal if shared else share all interned strings
 * (Larry,MAL) mark all constants unmarshalled for a module as immortal
 * (Larry,MAL) allocate (immutable) immortal objects in their own memory page(s)
+* saturated refcounts using the 32 least-significant bits
 
 Solutions for Accidental De-Immortalization
 -------------------------------------------
@@ -730,6 +749,9 @@ has further detail.)
 Regardless of the solution we end up with, we can do something else
 later if necessary.
 
+TODO: Add a note indicating that the implemented solution does not
+affect the overall ~performance-neutral~ outcome.
+
 Documentation
 -------------
 
@@ -784,6 +806,8 @@ References
 
 .. _Pyston: https://mail.python.org/archives/list/python-dev@python.org/message/JLHRTBJGKAENPNZURV4CIJSO6HI62BV3/
 
+.. _Facebook: https://www.facebook.com/watch/?v=437636037237097&t=560
+
 Prior Art
 ---------