[Python-Dev] CPython optimization: storing reference counters outside of objects

[Cesare Di Mauro]

2011/5/23 "Martin v. Löwis" <martin at v.loewis.de>

> > I'm not a compiler/profiling expert so the main question is if such
> > design can work, and maybe someone was thinking about something
> > similar?
>
> My expectation is that your approach would likely make the issues
> worse in a multi-CPU setting. If you put multiple reference counters
> into a contiguous block of memory, unrelated reference counters will
> live in the same cache line. Consequentially, changing one reference
> counter on one CPU will invalidate the cached reference counters of
> that cache line on other CPU, making your problem a) actually worse.
>
> Regards,
> Martin
>

I don't think that moving ob_refcnt to a proper memory pool will solve the
problem of cache pollution anyway.

ob_refcnt is obviously the most stressed field in PyObject, but it's not the
only one. We have , that is needed to model each object (instance)
"behavior", which is massively accessed too, so a cache line will be loaded
as well when the object will be used.

Also, only a few of simple objects have just ob_refcnt and ob_type. Most of
them have other fields too, and accessing them means a line cache load.

Regards,
Cesare

P.S. Memory allocation granularity can help sometimes, leaving some data
(ob_refcnt and/or ob_type) on one cache line, and the other on the next one.
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