使用未使用的变量可以提高 20% 的速度？为什么？

Question

我正在做很多基准测试。我从来没有见过这样的事情。我很困惑。创建一个根本不使用的额外全局变量可以使我的部分代码速度提高约 20%。 为什么？

我正在对一个生成可迭代对象的函数进行基准测试，测量消耗（迭代）它们所需的时间。我有两种食用方式。当我获得高 CPU 份额时的典型时间：

       | Without the variable | With the variable 
-------+----------------------+-------------------
Output |   0.74 s  consume_1  |  0.72 s  consume_1 
       |   0.96 s  consume_2  |  0.77 s  consume_2 
       |                      |                    
       |   0.74 s  consume_1  |  0.75 s  consume_1 
       |   0.96 s  consume_2  |  0.78 s  consume_2 
       |                      |                    
       |   0.73 s  consume_1  |  0.73 s  consume_1 
       |   0.95 s  consume_2  |  0.78 s  consume_2 
-------+----------------------+-------------------
Debug  |  Real time: 5.110 s  | Real time: 4.560 s
       |  User time: 4.546 s  | User time: 4.386 s
       |  Sys. time: 0.535 s  | Sys. time: 0.150 s
       |  CPU share: 99.43 %  | CPU share: 99.47 %

创建无意义的变量使 consume_2 的消耗速度加快了约 0.2 秒（从 0.97 到 0.77）。此外，“调试”统计数据也存在显着差异。最剧烈的是“系统时间”：对于“无”，它始终在 0.5 秒左右，对于“有”，它始终在 0.14 秒左右。

我正在 TIO 上执行此操作，您可以自己在那里重现它：
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这是代码，我将额外的变量称为 foobar。另请注意，consume_1 加载全局 deque 10000 次，而 consume_2 仅加载少量全局变量，所以如果有的话，我认为consume_1 将是受影响的。

from timeit import timeit
from operator import itemgetter
from itertools import repeat
from collections import deque

def each_with_others_1(iterable):
    xs = tuple(iterable)
    for i, x in enumerate(xs):
        yield x, xs[:i] + xs[i+1:]

consume_0 = None

def consume_1(each_with_others):
    for each, others in each_with_others:
        deque(others, 0)

def consume_2(each_with_others):
    otherss = map(itemgetter(1), each_with_others)
    deque(map(deque, otherss, repeat(0)), 0)

lst = list(range(10000))
foobar = None
for solver in [each_with_others_1] * 3:
    for consume in consume_1, consume_2:
        t = timeit(lambda: consume(solver(lst)), number=1)
        print('%.2f s ' % t, consume.__name__)
    print()

更新：在安装 Python 3.8.2 后也在 Google Compute Engine 实例上重现，创建变量使 consume_2 速度加快了约 15%：

       | Without the variable | With the variable 
-------+----------------------+-------------------
Output |   0.64 s  consume_1  |  0.65 s  consume_1 
       |   0.80 s  consume_2  |  0.68 s  consume_2 
       |                      |                    
       |   0.64 s  consume_1  |  0.65 s  consume_1 
       |   0.80 s  consume_2  |  0.68 s  consume_2 
       |                      |                    
       |   0.64 s  consume_1  |  0.64 s  consume_1 
       |   0.78 s  consume_2  |  0.68 s  consume_2 
-------+----------------------+-------------------
Debug  |   real   0m 4.327s   |  real   0m 3.987s
       |   user   0m 3.987s   |  user   0m 3.902s
       |   sys    0m 0.340s   |  sys    0m 0.084s

“调试”来自将其称为 time python test。 py.对于“without”，sys 始终在 0.32 秒左右。对于“with”，它始终在 0.09 秒左右。