c++ - 'managed_shared_memory' 应该分配多少内存？ ( boost )

Question

我正在寻找关于在通过 boost::interprocess 的 managed_shared_memory 创建静态共享内存块时应该分配多少内存的明确答案(如果确实存在的话) 。连official examples似乎分配arbitrarily large内存块。

考虑以下结构:

// Example: simple struct with two 4-byte fields
struct Point2D {
  int x, y;
};

我最初的 react 是必要的大小是 8 个字节，或 sizeof(Point2D)。当我尝试构造一个对象时，这惨遭失败，在运行时出现段错误。

// BAD: 8 bytes is nowhere near enough memory allocated.
managed_shared_memory segment(create_only, "My shared memory", sizeof(Point2D));

什么读/写操作导致段错误？堆栈操作？在 segment.construct() 中临时分配？分配共享内存时需要多少开销？

通过反复试验，我发现将大小乘以 4 可以用于上述结构，但是当我开始向我的 struct 添加更多字段时，它就会崩溃。所以，这有点像糟糕的黑客攻击。

有些人可能会争辩说现代 PC 中的“内存很便宜”，但我不同意这种理念，并且不喜欢分配比我需要的更多的内存，如果我能避免的话。我昨天浏览了 Boost 文档，但找不到任何建议。今天是为了学习新东西!

Answer 1

来自 this paragraph文档:

The memory algorithm is an object that is placed in the first bytes of a shared memory/memory mapped file segment.

内存段布局:

 ____________ __________ ____________________________________________  
|            |          |                                            | 
|   memory   | reserved |  The memory algorithm will return portions | 
| algorithm  |          |  of the rest of the segment.               | 
|____________|__________|____________________________________________| 

该库在段的开头有额外的内存开销，因此占用了您请求大小的几个字节。根据this post和 this post ，这个额外字节的确切数量无法确定:

You can't calculate it, because there are memory allocation bookeeping and fragmentation issues that change in runtime depending on your allocation/deallocation pattern. And shared memory is allocated by pages by the OS (4K on linux 64k on windows), so any allocation will be in practice allocated rounded to a page:

    managed_shared_memory segment(create_only, "name", 20);

will waste the same memory as:

    managed_shared_memory segment(create_only, "name", 4096);