[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build

[Victor Stinner]

Le mar. 9 avr. 2019 à 22:16, Steve Dower <steve.dower at python.org> a écrit :
> What are the other changes that would be required?

I don't know.

> And is there another
> way to get the same functionality without ABI modifications?

Py_TRACE_REFS is a double linked list of *all* Python objects. To get
this functionality, you need to store the list somewhere. I don't know
how to maintain such list outside the PyObject structure.

One solution would be to enable Py_TRACE_REFS in release mode. Does
anyone want to add 16 bytes to every PyObject? I don't want that :-)


> I think it's worthwhile if we can really get to debug and non-debug
> builds being ABI compatible. Getting partway there in this case doesn't
> seem to offer any benefits.

Disabling Py_TRACE_REFS by default in debug mode reduces the Python
memory footprint. Py_TRACE_REFS costs 2 pointers per PyObject: 16
bytes on 64-bit platforms.

I don't think that I ever used sys.getobjects(), whereas many projects
use gc.get_objects() which is also available in release builds (not
only in debug builds).

I'm quite sure that almost nobody uses debug builds because the ABI is
incompatible.

The main question is if anyone ever used Py_TRACE_REFS? Does someone
use sys.getobjects() or PYTHONDUMPREFS environment variable?

Using PYTHONDUMPREFS=1 on a debug build (with Py_TRACE_REFS) does
simply crash Python 3.7 at exit. So I don't think that anyone use it
:-)


I wrote PR 12614 to remove all code related to Py_TRACE_REFS. I wrote
it to see which code depends on it:

commit 63509498761a0e7f72585a8cd7df325ea2abd1b2 (HEAD ->
remove_trace_refs, origin/remove_trace_refs)
Author: Victor Stinner <vstinner at redhat.com>
Date:   Thu Mar 28 23:26:58 2019 +0100

    WIP: bpo-36465: Remove Py_TRACE_REFS special build

    Remove _ob_prev and _ob_next fields of PyObject when Python is
    compiled in debug mode to make debug ABI closer to the release ABI.

    Remove:

    * sys.getobjects()
    * PYTHONDUMPREFS environment variable
    * _PyCoreConfig.dump_refs
    * PyObject._ob_prev and PyObject._ob_next fields
    * _PyObject_HEAD_EXTRA and _PyObject_EXTRA_INIT macros
    * _Py_AddToAllObjects()
    * _Py_PrintReferenceAddresses()
    * _Py_PrintReferences()

Victor
-- 
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