malloc: unused heap is still kept in heap list

[Shuo Wang]

Hi,
According to heap_trim, heap will be freed from the the 
one where the top chunk is located in. (We will call it
last heap below)

static int
heap_trim (heap_info *heap, size_t pad) 
{
  while (top_chunk == chunk_at_offset (heap, sizeof (*heap)))
    {
        ...
        delete_heap();
        ...
    }
}


As shown below, however, when last heap is still used 
prev heap will not be freed to os even if there are just 
unused chunks in prev heap.

|---------------------|       |---------------------|
|       top chunk     |       |                     |
|---------------------|       |                     |
|   allocated chunk   |       |                     |
|---------------------|       |                     |
|         ...         |       |      free chunk     |
|---------------------|       |                     |
|     free chunk      |       |                     |
|---------------------|       |                     |
|   allocated chunk   |       |                     |
|---------------------|       |---------------------|
|                     |       |                     |
|      heap info      |  ---> |      heap info      |
|                     |       |                     |
|---------------------|       |---------------------|

I was wondering if you can explain that why we keep the free heap in heap list.
And how can we free this kind of heap when our service is running.

Cheers,
Shuo