std::deque 的内存开销是怎么回事?

问题描述

我正在研究使用 std::queue 的外部排序算法，并且必须仔细限制其内存使用.我注意到在合并阶段(使用几个固定长度的 std::queue )，我的内存使用量增加到我预期的大约 2.5 倍.由于 std::queue 默认使用 std::deque 作为其底层容器，我对 std::deque 进行了一些测试以确定其内存开销.以下是在 VC++ 9 上运行的结果，在发布模式下，使用 64 位进程:

I am working on an external sorting algorithm that uses std::queue and must carefully constrain its memory usage. I have noticed that during the merge phase (which uses several std::queues of fixed length), my memory usage increases to about 2.5X what I expected. Since std::queue by default uses std::deque as its underlying container, I ran some tests on std::deque to determine its memory overhead. Here are the results, running on VC++ 9, in release mode, with a 64-bit process:

当向 std::deque 添加 100,000,000 个 char 时，内存使用量增长到 252,216K.请注意，100M chars(1 字节)应占用 97,656K，因此这是 154,560K 的开销.

When adding 100,000,000 chars to a std::deque, the memory usage grows to 252,216K. Note that 100M chars (1 byte) should occupy 97,656K, so this is an overhead of 154,560K.

我用 doubles(8 字节)重复了测试，看到内存增长到 1,976,676K，而 100M doubles 应该占用 781,250K，开销为 1,195,426K！！

I repeated the test with doubles (8 bytes) and saw memory grow to 1,976,676K, while 100M doubles should occupy 781,250K, for an overhead of 1,195,426K!!

现在我明白 std::deque 通常被实现为块"的链表.如果这是真的，那么为什么开销与元素大小成正比(因为当然指针大小应该固定为 8 个字节)?为什么它如此危险?

Now I understand that std::deque is normally implemented as a linked list of "chunks." If this is true, then why is the overhead proportional to the element size (because of course the pointer size should be fixed at 8 bytes)? And why is it so danged huge?

谁能解释一下为什么 std::deque 使用这么多危险内存?我想我应该将我的 std::queue 底层容器切换到 std::vector 因为没有开销(假设适当的大小是 reserveed).我认为 std::deque 的好处在很大程度上被这样一个事实否定了，因为它有如此巨大的开销(导致缓存未命中、页面错误等)，并且复制成本std::vector 元素可能会更少，因为整体内存使用量要低得多.这只是微软对 std::deque 的错误实现吗?

Can anybody shed some light on why std::deque uses so much danged memory? I'm thinking I should switch my std::queue underlying containers to std::vector as there is no overhead (given that the appropriate size is reserveed). I'm thinking the benefits of std::deque are largely negated by the fact that it has such a huge overhead (resulting in cache misses, page faults, etc.), and that the cost of copying std::vector elements may be less, given that the overall memory usage is so much lower. Is this just a bad implementation of std::deque by Microsoft?

推荐答案

看代码_DEQUESIZ(每块的元素数):

Look at the code for _DEQUESIZ (number of elements per block):

#define _DEQUESIZ   (sizeof (_Ty) <= 1 ? 16 
    : sizeof (_Ty) <= 2 ? 8 
    : sizeof (_Ty) <= 4 ? 4 
    : sizeof (_Ty) <= 8 ? 2 : 1)    /* elements per block (a power of 2) */

如果元素变大，它会变小.只有对于大于 8 字节的元素，您才会获得预期的行为(随着元素大小的增加，开销百分比降低).

It gets smaller if the element is larger. Only for elements larger than 8 bytes will you get the expected behavior (percentual decrease of overhead with increase of element size).

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