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

[Nathaniel Smith]

On Mon, Apr 15, 2019 at 8:58 PM Michael Sullivan <sully at msully.net> wrote:
>
> On Mon, Apr 15, 2019 at 4:06 PM Nathaniel Smith <njs at pobox.com> wrote:
>>
>> On Mon, Apr 15, 2019, 15:27 Michael Sullivan <sully at msully.net> wrote:
>>>
>>> > The main question is if anyone ever used Py_TRACE_REFS? Does someone
>>> > use sys.getobjects() or PYTHONDUMPREFS environment variable?
>>>
>>> I used sys.getobjects() today to track down a memory leak in the mypyc-compiled version of mypy.
>>>
>>> We were leaking memory badly but no sign of the leak was showing up in mypy's gc.get_objects() based profiler. Using a debug build and switching to sys.getobjects() showed that we were badly leaking int objects. A quick inspection of the values in question (large and random looking) suggested we were leaking hash values, and that quickly pointed me to https://github.com/mypyc/mypyc/pull/562.
>>>
>>> I don't have any strong feelings about whether to keep it in the "default" debug build, though. I was using a debug build that I built myself with every debug feature that seemed potentially useful.
>>
>>
>> This is mostly to satisfy my curiosity, so feel free to ignore: did you try using address sanitizer or valgrind?
>>
> I didn't, mostly because I assume that valgrind wouldn't play well with cpython. (I've never used address sanitizer.)
>
> I was curious, so I went back and tried it out.
> It turned out to not seem to need that much fiddling to get to work. It slows things down a *lot* and produced 17,000 "loss records", though, so maybe I don't have it working right. At a glance the records did not shed any light.
>
> I'd definitely believe that valgrind is up to the task of debugging this, but my initial take with it shed much less light than my sys.getobjects() approach. (Though note that my sys.getobjects() approach was slotting it into an existing python memory profiler we had hacked up, so...)

valgrind on CPython is definitely a bit fiddly – if you need it again
you might check out Misc/README.valgrind.

Supposedly memory sanitizer is just './configure
--with-memory-sanitizer', but I haven't tried it either :-)

-n

-- 
Nathaniel J. Smith -- https://vorpus.org