[Python-Dev] New calling convention to avoid temporarily tuples when calling functions

[Victor Stinner]

Oh, I found a nice pice of CPython history in Modules/_pickle.c.
Extract of Python 3.3:
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/* A temporary cleaner API for fast single argument function call.

   XXX: Does caching the argument tuple provides any real performance benefits?

   A quick benchmark, on a 2.0GHz Athlon64 3200+ running Linux 2.6.24 with
   glibc 2.7, tells me that it takes roughly 20,000,000 PyTuple_New(1) calls
   when the tuple is retrieved from the freelist (i.e, call PyTuple_New() then
   immediately DECREF it) and 1,200,000 calls when allocating brand new tuples
   (i.e, call PyTuple_New() and store the returned value in an array), to save
   one second (wall clock time). Either ways, the loading time a pickle stream
   large enough to generate this number of calls would be massively
   overwhelmed by other factors, like I/O throughput, the GC traversal and
   object allocation overhead. So, I really doubt these functions provide any
   real benefits.

   On the other hand, oprofile reports that pickle spends a lot of time in
   these functions. But, that is probably more related to the function call
   overhead, than the argument tuple allocation.

   XXX: And, what is the reference behavior of these? Steal, borrow? At first
   glance, it seems to steal the reference of 'arg' and borrow the reference
   of 'func'. */
static PyObject *
_Pickler_FastCall(PicklerObject *self, PyObject *func, PyObject *arg)
-----------------

Extract of Python 3.4 (same function):
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    /* Note: this function used to reuse the argument tuple. This used to give
       a slight performance boost with older pickle implementations where many
       unbuffered reads occurred (thus needing many function calls).

       However, this optimization was removed because it was too complicated
       to get right. It abused the C API for tuples to mutate them which led
       to subtle reference counting and concurrency bugs. Furthermore, the
       introduction of protocol 4 and the prefetching optimization via peek()
       significantly reduced the number of function calls we do. Thus, the
       benefits became marginal at best. */
-----------------

It recalls me the story of property_descr_get() optimizations :-)

I hope that the new generic "fastcall" functions will provide a safe
and reliable optimization for the pickle module, property_descr_get()
and others optimized functions.

Victor