[issue39511] [subinterpreters] Per-interpreter singletons (None, True, False, etc.)
Jeremy Kloth
report at bugs.python.org
Sat Feb 1 06:23:50 EST 2020
Jeremy Kloth <jeremy.kloth at gmail.com> added the comment:
> The problem is to make Py_INCREF/Py_DECREF efficient.
That is exactly why I didn't propose a change to them. The singletons
still are refcounted as usual, just that their ob_refcnt is ignored.
If they somehow reach 0, they just "resurrect" themselves and ignore
the regular collection behavior. In the presence of multiple
DECREF'ers, the ob_refcnt field is garbage, but that is OK as it is
effectively ignored. Practicality vs Purity and all that.
> Last time someone tried to use an atomic variable for ob_refcnt, it was 20% slower if I recall correctly. If many threads start to update such atomic variable, the CPU cacheline of common singletons like None, True and False can quickly become a performance bottleneck.
Exactly so, hence why I chose the simple solution of effectively
ignoring ob_refcnt.
> On the other side, if each interpreter has its own objects, there is no need to protect ob_refcnt, the interpreter lock protects it.
My solution also does not need any protection around ob_refcnt.
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nosy: +jeremy.kloth
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