rust 的内存管理

在RUST的文档说明中可以看到：

RUST认为引用计数以及其它的收回堆内存在方式存在效率低下及功耗过大方面的问题。因此它默认采用LIFO，即采用栈空间的方式来管理内存。以下为文档原文。

What do other languages do?

Most languages with a garbage collector heap-allocate by default. This means that every value is boxed. There are a number of reasons why this is done, but they’re out of scope for this tutorial. There are some possible optimizations that don’t make it true 100% of the time, too. Rather than relying on the stack andDrop to clean up memory, the garbage collector deals with the heap instead.

Which to use?

So if the stack is faster and easier to manage, why do we need the heap? A big reason is that Stack-allocation alone means you only have LIFO semantics for reclaiming storage. Heap-allocation is strictly more general, allowing storage to be taken from and returned to the pool in arbitrary order, but at a complexity cost.

Generally, you should prefer stack allocation, and so, Rust stack-allocates by default. The LIFO model of the stack is simpler, at a fundamental level. This has two big impacts: runtime efficiency and semantic impact.

Runtime Efficiency

Managing the memory for the stack is trivial: The machine just increments or decrements a single value, the so-called “stack pointer”. Managing memory for the heap is non-trivial: heap-allocated memory is freed at arbitrary points, and each block of heap-allocated memory can be of arbitrary size, the memory manager must generally work much harder to identify memory for reuse.

If you’d like to dive into this topic in greater detail, this paper is a great introduction.

Semantic impact

Stack-allocation impacts the Rust language itself, and thus the developer’s mental model. The LIFO semantics is what drives how the Rust language handles automatic memory management. Even the deallocation of a uniquely-owned heap-allocated box can be driven by the stack-based LIFO semantics, as discussed throughout this chapter. The flexibility (i.e. expressiveness) of non LIFO-semantics means that in general the compiler cannot automatically infer at compile-time where memory should be freed; it has to rely on dynamic protocols, potentially from outside the language itself, to drive deallocation (reference counting, as used by Rc<T> and Arc<T>, is one example of this).

When taken to the extreme, the increased expressive power of heap allocation comes at the cost of either significant runtime support (e.g. in the form of a garbage collector) or significant programmer effort (in the form of explicit memory management calls that require verification not provided by the Rust compiler).

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1. We can make the memory live longer by transferring ownership, sometimes called ‘moving out of the box’. More complex examples will be covered later. ↩