这个问题可能听起来相当简单,但这是我与另一位开发人员的争论。

我小心地在我可以的地方进行堆栈分配,而不是堆分配。他和我说话,看着我的肩膀,并评论说,这是没有必要的,因为他们的表现是一样的。

在我的印象中,增加堆栈是一个常数时间,而堆分配的性能取决于当前堆的复杂性,包括分配(找到合适大小的孔)和反分配(缩小孔以减少碎片,如果我没有弄错的话,许多标准库实现在删除过程中需要时间来做这件事)。

在我看来,这可能非常依赖于编译器。在这个项目中,我特别使用了一个用于PPC架构的Metrowerks编译器。对这种组合的深入了解将是最有帮助的,但一般来说,对于GCC和msvc++,情况如何?堆分配不如堆栈分配高效吗?没有区别吗?还是差异如此之小以至于变成了毫无意义的微观优化。


当前回答

除了与堆分配相比具有数量级的性能优势外,堆栈分配对于长时间运行的服务器应用程序更可取。即使是管理得最好的堆最终也会碎片化,导致应用程序性能下降。

其他回答

堆栈分配要快得多,因为它所做的只是移动堆栈指针。 使用内存池,您可以从堆分配中获得类似的性能,但这会略微增加复杂性,并带来令人头痛的问题。

此外,堆栈与堆不仅是性能方面的考虑;它还告诉您许多关于对象的预期生存期的信息。

您可以为特定大小的对象编写一个非常高性能的特殊堆分配器。但是,一般的堆分配器性能不是特别好。

我也同意Torbjörn Gyllebring关于对象的预期生命期的观点。好点!

Remark that the considerations are typically not about speed and performance when choosing stack versus heap allocation. The stack acts like a stack, which means it is well suited for pushing blocks and popping them again, last in, first out. Execution of procedures is also stack-like, last procedure entered is first to be exited. In most programming languages, all the variables needed in a procedure will only be visible during the procedure's execution, thus they are pushed upon entering a procedure and popped off the stack upon exit or return.

现在来看一个不能使用堆栈的例子:

Proc P
{
  pointer x;
  Proc S
  {
    pointer y;
    y = allocate_some_data();
    x = y;
  }
}

If you allocate some memory in procedure S and put it on the stack and then exit S, the allocated data will be popped off the stack. But the variable x in P also pointed to that data, so x is now pointing to some place underneath the stack pointer (assume stack grows downwards) with an unknown content. The content might still be there if the stack pointer is just moved up without clearing the data beneath it, but if you start allocating new data on the stack, the pointer x might actually point to that new data instead.

通常,堆栈分配只是由堆栈指针寄存器中的减法组成。这比搜索堆快多了。

Sometimes stack allocation requires adding a page(s) of virtual memory. Adding a new page of zeroed memory doesn't require reading a page from disk, so usually this is still going to be tons faster than searching a heap (especially if part of the heap was paged out too). In a rare situation, and you could construct such an example, enough space just happens to be available in part of the heap which is already in RAM, but allocating a new page for the stack has to wait for some other page to get written out to disk. In that rare situation, the heap is faster.

堆栈的容量有限,而堆则不是。一个进程或线程的典型堆栈大约是8K。一旦分配,就不能更改大小。

堆栈变量遵循作用域规则,而堆变量则不遵循。如果你的指令指针超出了一个函数,所有与该函数相关的新变量都会消失。

最重要的是,您无法预先预测整个函数调用链。因此,仅200字节的分配就可能导致堆栈溢出。如果您正在编写一个库,而不是应用程序,这一点尤其重要。