为什么在 python3.4 和 python2.7 下 hash() 更慢 [英] Why is hash() slower under python3.4 vs python2.7

问题描述

我正在使用 timeit 进行一些性能评估，并发现 python 2.7.10 和 python 3.4.3 之间的性能下降.我把它缩小到 hash() 函数:

蟒蛇 2.7.10:

<预><代码>>>>导入时间>>>timeit.timeit('for x in xrange(100): hash(x)', number=100000)0.4529099464416504>>>timeit.timeit('hash(1000)')0.044638872146606445

蟒蛇 3.4.3:

<预><代码>>>>导入时间>>>timeit.timeit('for x in range(100): hash(x)', number=100000)0.6459149940637872>>>timeit.timeit('hash(1000)')0.07708719989750534

那是大约.降级40%！整数、浮点数、字符串(unicodes 或 bytearrays)等是否被散列似乎并不重要；退化程度大致相同.在这两种情况下，哈希都返回一个 64 位整数.以上是在我的 Mac 上运行的，在 Ubuntu 机器上降级较小(20%).

我还在 python2.7 测试中使用了 PYTHONHASHSEED=random，在某些情况下，为每个案例"重新启动 python，我看到了 hash()性能变差了一点，但永远不会像python3.4一样慢

有人知道这是怎么回事吗?是否为 python3 选择了更安全但速度更慢的哈希函数?

解决方案

hash() 函数在 Python 2.7 和 Python 3.4 之间有两个变化

1. 采用 SipHash
2. 默认启用哈希随机化

<小时>

参考:

• 从 Python 3.4 开始，它使用 SipHash 作为散列函数.阅读:Python 采用 SipHash
• 自 Python 3.3 哈希随机化默认启用. 参考:object.__hash__(本节的最后一行).指定 PYTHONHASHSEED 值 0 将禁用哈希随机化.

I was doing some performance evaluation using timeit and discovered a performance degredation between python 2.7.10 and python 3.4.3. I narrowed it down to the hash() function:

python 2.7.10:

>>> import timeit
>>> timeit.timeit('for x in xrange(100): hash(x)', number=100000)
0.4529099464416504
>>> timeit.timeit('hash(1000)')
0.044638872146606445

python 3.4.3:

>>> import timeit
>>> timeit.timeit('for x in range(100): hash(x)', number=100000)
0.6459149940637872
>>> timeit.timeit('hash(1000)')
0.07708719989750534

That's an approx. 40% degradation! It doesn't seem to matter if integers, floats, strings(unicodes or bytearrays), etc, are being hashed; the degradation is about the same. In both cases the hash is returning a 64-bit integer. The above was run on my Mac, and got a smaller degradation (20%) on an Ubuntu box.

I've also used PYTHONHASHSEED=random for the python2.7 tests and in some cases, restarting python for each "case", I saw the hash() performance get a bit worse, but never as slow as python3.4

Anyone know what's going on here? Was a more-secure, but slower, hash function chosen for python3 ?

解决方案

There are two changes in hash() function between Python 2.7 and Python 3.4

1. Adoptions of SipHash
2. Default enabling of Hash randomization

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References:

• Since from Python 3.4, it uses SipHash for it's hashing function. Read: Python adopts SipHash
• Since Python 3.3 Hash randomization is enabled by default. Reference: object.__hash__ (last line of this section). Specifying PYTHONHASHSEED the value 0 will disable hash randomization.

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