Why is memory allocation on heap MUCH slower than on stack?

Question

I have been told this many times. But I don\'t know WHY...What extra cost is involved when allocating memory from heap? Is it hardware related? Is it related to CPU cycles?

Answer 1

The main difference between a stack and a heap is that items on a stack cannot be removed out of order. If you add items A, B, C to a stack, you can't remove B without removing C first. This means that adding a new item to a stack always means adding it to the end of the stack, which is a very simple operation. You just move the pointer that points to the end of the stack.

On a heap on the other hand, you can remove items out of order. And as long as you don't move the other items around afterwards in memory (as some garbage collected heaps do), your heap then has "hole" in the middle. I.e. if you add A,B,C to a heap and remove B, your heap looks like this in memory: A _ C where _ is a block of unused (free) memory. If you add a new item D now, the allocator has to find a continuous free space big enough to fit D. Depending on how many continuous free spaces there are in your memory, this can be an expensive operation. And it's almost always more expensive than just moving the "last element" pointer of a stack.