提交 318072c2 编写于 作者: S sneaxiy

add comments of allocator design

test=develop
上级 2d92b6be
......@@ -46,10 +46,49 @@ class Allocator;
// NOTE: this is the base class of Allocation. Each allocator can use its own
// allocation object.
// NOTE: the `Allocation::ptr()` could be nullptr, if the allocation size is 0
/**
* Allocation is returned by Allocator::Allocate() method.
*
* An allocator may be decorated by another allocator. For example, we can
* decorate
* a RetryAllocator to any allocator to perform allocation retry when first
* allocation request fails.
*
* Explanations of Allocator design is as follows:
*
* Suppose we have an allocator which is decorated by several allocators:
*
* A(1) <- A(2) <- A(3) <- ... <- A(n)
*
* , and the public allocator is A(1).
*
* The allocation process would be:
*
* A(n).Allocate() -> ... -> A(2).Allocate() -> A(1).Allocate()
*
* , and the free process would be:
*
* A(1).Free() -> A(2).Free() -> ... -> A(n).Free()
*
* Therefore, we should record the allocator chain when allocating, so
* that we can free the allocation in the reverse order of allocator chain.
* The field `decorated_allocators_` is used to record this chain.
*
* Another example is that we want to add additional fields in Allocation,
* e.g., something what is done in AlignedAllocator, etc.
* In this case, we should declare a derived class of Allocation, which
* contains an underlying Allocation allocated by the underlying allocator.
* Therefore, `decorated_allocators_` of the new Allocation object would
* be a new chain, differing from the underlying Allocation object.
*/
class Allocation {
public:
Allocation(void* ptr, size_t size, platform::Place place)
: ptr_(ptr), size_(size), place_(place) {
// NOTE(zjl): Since decorated_allocators_ is usually a small vector
// We reserve a small buffer to it to prevent frequent heap allocation
// Not quite sure whether we need something like gtl vector.
decorated_allocators_.reserve(8);
}
......
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