sys.getsizeof（）结果与结构大小不相关 [英] sys.getsizeof() results don't quite correlate to structure size

问题描述

我正在尝试创建大小为1 MB的列表。而以下代码可以工作：

I am trying to create a list of size 1 MB. while the following code works:

dummy = ['a' for i in xrange(0, 1024)]
sys.getsizeof(dummy)
Out[1]: 9032

以下代码不工作。

import os
import sys

dummy = []
dummy.append((os.urandom(1024))
sys.getsizeof(dummy)
Out[1]: 104

有人可以解释为什么吗？

Can someone explain why?

如果你想知道为什么我不使用第一个代码片段，我正在写一个程序通过编写一个写入块（大小为1 B，1 KB和1 MB）的内存循环来对内存进行基准测试。

If you're wondering why I am not using the first code snippet, I am writing a program to benchmark my memory by writing a for loop that writes blocks (of size 1 B, 1 KB and 1 MB) into memory.

start = time.time() 
for i in xrange(1, (1024 * 10)):  
     dummy.append(os.urandom(1024)) #loop to write 1 MB blocks into memory
end = time.time()

推荐答案

如果您检查列表的大小，它将提供列表数据结构的大小，包括t他指向其组成元素。它不会考虑元素的大小。

If you check the size of a list, it will be provide the size of the list data structure, including the pointers to its constituent elements. It won't consider the size of elements.

str1_size = sys.getsizeof(['a' for i in xrange(0, 1024)])
str2_size = sys.getsizeof(['abc' for i in xrange(0, 1024)])
int_size = sys.getsizeof([123 for i in xrange(0, 1024)])
none_size = sys.getsizeof([None for i in xrange(0, 1024)])
str1_size == str2_size == int_size == none_size

空列表的大小： sys.getsizeof（[]）== 72

添加元素： sys.getsizeof（[1]）== 80

添加另一个元素： sys.getsizeof（[1，1]）== 88

所以每个元素添加4个字节。

要获取1024个字节，我们需要code>（1024 - 72）/ 8 = 119 元素。

The size of empty list: sys.getsizeof([]) == 72
Add an element: sys.getsizeof([1]) == 80
Add another element: sys.getsizeof([1, 1]) == 88
So each element adds 4 bytes.
To get 1024 bytes, we need (1024 - 72) / 8 = 119 elements.

列表中有119个元素的大小： sys.getsizeof（[在xrange（0，119）中没有）] == 1080 。

这是因为一个列表维护一个额外的缓冲区来插入更多的项目，所以每次都不需要调整大小。 （107和126之间的元素的大小与1080相同）

The size of the list with 119 elements: sys.getsizeof([None for i in xrange(0, 119)]) == 1080.
This is because a list maintains an extra buffer for inserting more items, so that it doesn't have to resize every time. (The size comes out to be same as 1080 for number of elements between 107 and 126).

所以我们需要的是不可变数据结构，不需要保持这个缓冲区 - tuple 。

So what we need is an immutable data structure, which doesn't need to keep this buffer - tuple.

empty_tuple_size = sys.getsizeof(())                     # 56
single_element_size = sys.getsizeof((1,))                # 64
pointer_size = single_element_size - empty_tuple_size    # 8
n_1mb = (1024 - empty_tuple_size) / pointer_size         # (1024 - 56) / 8 = 121
tuple_1mb = (1,) * n_1mb
sys.getsizeof(tuple_1mb) == 1024

所以这是您获得1MB数据结构的答案：（1，）* 121

So this is your answer to get a 1MB data structure: (1,)*121

但请注意，这只是元组和组成指针的大小。对于总大小，实际上需要将各个元素的大小相加。

But note that this is only the size of tuple and the constituent pointers. For the total size, you actually need to add up the size of individual elements.

替代：

sys.getsizeof('') == 37
sys.getsizeof('1') == 38     # each character adds 1 byte

对于1 MB，我们需要987个字符：

For 1 MB, we need 987 characters:

sys.getsizeof('1'*987) == 1024

这是实际大小，而不仅仅是指针的大小。

And this is the actual size, not just the size of pointers.

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