I use the test case of https://bugs.python.org/issue1635741#msg355187 to test the refleaks in debug mode.

Before this PR:
sys.gettotalrefcount: 14288
sys.gettotalrefcount: 18042
sys.gettotalrefcount: 21796
sys.gettotalrefcount: 25550
sys.gettotalrefcount: 29304
sys.gettotalrefcount: 33058
sys.gettotalrefcount: 36812
sys.gettotalrefcount: 40566
sys.gettotalrefcount: 44320
sys.gettotalrefcount: 48074

After this PR:
sys.gettotalrefcount: 13641
sys.gettotalrefcount: 16748
sys.gettotalrefcount: 19855
sys.gettotalrefcount: 22962
sys.gettotalrefcount: 26069
sys.gettotalrefcount: 29176
sys.gettotalrefcount: 32283
sys.gettotalrefcount: 35390
sys.gettotalrefcount: 38497
sys.gettotalrefcount: 41604

@vstinner Hi, victor. Pls take a look if you have free time, thanks.

Maybe you're talking about a "reference cycle".

Thanks, victor. "reference cycle" would be more exact. And I will try your idea in my interpreter.

    /* Trigger a GC collection on subinterpreters*/
    if (!is_main_interp) {
        _PyGC_CollectNoFail();
    }

with:

    // Last explicit GC collection
    _PyGC_CollectNoFail();

Oh, amazing result:

sys.gettotalrefcount: 10537
sys.gettotalrefcount: 10540
sys.gettotalrefcount: 10543
sys.gettotalrefcount: 10546
sys.gettotalrefcount: 10549
sys.gettotalrefcount: 10552
sys.gettotalrefcount: 10555
sys.gettotalrefcount: 10558
sys.gettotalrefcount: 10561
sys.gettotalrefcount: 10564

the pr in: #21902

Pablo created this PR(don't calling explict collection in main interpreter): #17457
I am not sure I missed some info or not~

Since #17457 is merged, is this PR still relevant/useless? If not, please close it.

Since #17457 is merged, is this PR still relevant/useless? If not, please close it.

#17457 is worked, thanks.