为什么.sum()比.any()或.max()快? [英] Why is .sum() faster than .any() or .max()?

问题描述

优化代码的慢速部分时，我惊讶于A.sum()几乎是A.max()的两倍:

When optimising slow parts of my code, I was surprised by the fact that A.sum() is almost twice as fast as A.max():

In [1]: A = arange(10*20*30*40).reshape(10, 20, 30, 40)

In [2]: %timeit A.max()
1000 loops, best of 3: 216 us per loop

In [3]: %timeit A.sum()
10000 loops, best of 3: 119 us per loop

In [4]: %timeit A.any()
1000 loops, best of 3: 217 us per loop

我曾期望A.any()会更快(它只需要检查一个元素！)，其次是A.max()，而A.sum()则是最慢的(sum()需要加数字和每次更新一个值，max每次都需要比较数字，有时需要更新，我认为加法应该比比较慢.实际上，情况恰恰相反.为什么?

I had expected that A.any() would be much faster (it should need to check only one element!), followed by A.max(), and that A.sum() would be the slowest (sum() needs to add numbers and update a value every time, max needs to compare numbers every time and update sometimes, and I thought adding should be slower than comparing). In fact, it's the opposite. Why?

推荐答案

max必须存储一个值，连续检查可能的更新(并且CPU需要执行分支操作才能实现这些更新). sum只会搅动这些值.

max has to store a value, continuously checking for potential updates (and the CPU needs to do branche operations to effect these). sum just churns through the values.

所以sum会更快.

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