[Python-Dev] iterzip()

2002年4月29日 16:03:48 -0400

>>>>> "TP" == Tim Peters <tim.one@comcast.net> writes:

 TP> Check this out:
 TP> """ from time import clock as now
 TP> n = 1000000
 TP> def juststore(x):
 TP> for i in x: x[i] = 0
 TP> def justtups(x):
 TP> for i in x: 0,
 TP> def storetups(x):
 TP> for i in x: x[i] = 0,
 TP> def run(func):
 TP> x = range(n) start = now() func(x) finish = now() print
 TP> "%10s %6.2f" % (func.__name__, finish - start)
 TP> run(juststore) run(justtups) run(storetups) """
 TP> I get:
 TP> juststore 0.93
 TP> justtups 0.58
 TP> storetups 7.61
 TP> list.append is out of the picture here. Creating a million
 TP> tuples goes fast so long as they're recycled, and storing a
 TP> million things goes fast, but storing a million distinct tuples
 TP> takes very much longer. It's a Mystery to me so far. How does
 TP> it act on Linux?
It acts about the same on Linux.
I don't see why it's a mystery. justtups() only uses one tuple at a
time; it gets re-used from the free list every time. storetups() has
to (py)malloc a new tuple every stinkin' time.
Note that there's a net excess of allocations in the storetup() case,
so we invoke the garbage collector every n/700 times through the
loop. I've noted before that it doesn't make much sense to invoke GC
unless there is at least one deallocation; you can't reclaim anything
if there are no DECREFs.
Jeremy