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[Python-Dev] Investigating Python memory footprint of one real Web application

Thomas Wouters thomas at python.org
Tue Jan 24 06:33:27 EST 2017 
On Fri, Jan 20, 2017 at 1:40 PM, Christian Heimes <christian at python.org>
wrote:

> On 2017-01-20 13:15, INADA Naoki wrote:
> >>
> >> "this script counts static memory usage. It doesn’t care about dynamic
> >> memory usage of processing real request"
> >>
> >> You may be trying to optimize something which is only a very small
> >> fraction of your actual memory footprint.  That said, the marshal
> >> module could certainly try to intern some tuples and other immutable
> >> structures.
> >>
> >
> > Yes.  I hadn't think static memory footprint is so important.
> >
> > But Instagram tried to increase CoW efficiency of prefork application,
> > and got some success about memory usage and CPU throughput.
> > I surprised about it because prefork only shares static memory footprint.
> >
> > Maybe, sharing some tuples which code object has may increase cache
> efficiency.
> > I'll try run pyperformance with the marshal patch.
>
> IIRC Thomas Wouters (?) has been working on a patch to move the ref
> counter out of the PyObject struct and into a dedicated memory area. He
> proposed the idea to improve cache affinity, reduce cache evictions and
> to make CoW more efficient. Especially modern ccNUMA machines with
> multiple processors could benefit from the improvement, but also single
> processor/multi core machines.
>

FWIW, I have a working patch for that (against trunk a few months back,
even though the original idea was for the gilectomy branch), moving just
the refcount and not PyGC_HEAD. Performance-wise, in the benchmarks it's a
small but consistent loss (2-5% on a noisy machine, as measured by
python-benchmarks, not perf), and it breaks the ABI as well as any code
that dereferences PyObject.ob_refcnt directly (the field was repurposed and
renamed, and exposed as a const* to avoid direct assignment). It also
exposes the API awkwardness that CPython doesn't *require* objects to go
through a specific mechanism for object initialisation, even though nearly
all extension modules do so. (That same API awkwardness made life a little
harder when experimenting with BDW GC :P.) I don't believe external
refcounts can be made the default without careful redesigning of a new set
of PyObject API calls and deprecation of the old ones.

-- 
Thomas Wouters <thomas at python.org>

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