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Figure 64 If an allocation request of size 8 arrives at the back end allocator, the heap manager first consults the free lists In order to maximize efficiency when looking for free heap blocks, the heap manager keeps a free list bitmap The bitmap consists of 128 bits, where each bit represents an index into the free list table If the bit is set, the free list
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corresponding to the index of the free list bitmap contains free heap blocks Conversely, if the bit is not set, the free list at that index is empty Figure 65 shows the free list bitmap for the free lists in Figure 64
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0 1 1 0 2 1 3 0 4 0 5 0
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Figure 65 The heap manager maps an allocation request of a given size to a free list bitmap index by adding 8 bytes to the size (metadata) and dividing by 8 Consider an allocation request of size 8 bytes The heap manager knows that the free list bitmap index is 2 [(8+8)/8] From Figure 65, we can see that index 2 of the free list bitmap is set, which indicates that the free list located at index 2 in the free lists table contains free heap blocks The free block is then removed from the free list and returned to the caller If the removal of a free heap block results in that free list becoming empty, the heap manager also clears the free list bitmap at the specific index If the heap manager is unable to find a free heap block of requested size, it employs a technique known as block splitting Block splitting refers to the heap manager s capability to take a larger than requested free heap block and split it in half to satisfy a smaller allocation request For example, if an allocation request arrives for a block of size 8 (total block size of 16), the free list bitmap is consulted first The index representing blocks of size 16 indicates that no free blocks are available Next, the heap manager finds that free blocks of size 32 are available The heap manager now removes a block of size 32 and splits it in half, which yields two blocks of size 16 each One of the blocks is put into a free list representing blocks of size 16, and the other block is returned to the caller Additionally, the free list bitmap is updated to indicate that index 2 now contains free block entries of size 16 The result of splitting a larger free allocation into two smaller allocations is shown in Figure 66 As mentioned earlier, the free list at index 0 can contain free heap blocks of sizes ranging from 1016 up to 0x7FFF0 (524272) bytes To maximize free block lookup efficiency, the heap manager stores the free blocks in sorted order (ascending) All allocations of sizes greater than 0x7FFF0 go on what is known as the virtual allocation list When a large allocation occurs, the heap manager makes an explicit allocation request from the virtual memory manager and keeps these allocations on the virtual allocation list
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Free Lists 0 1 2 3 4 127 32 32 Step 1: First block of size 32 is split into two 16 byte blocks and removed from the free list 16 Step 2: One 16 byte block is added to the free list and one is returned to caller 1200 2100 2300
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Step 3: Free list bitmap updated to reflect changes after block splitting
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Figure 66 So far, the discussion has revolved around how the heap manager organizes blocks of memory it has at its disposal One question remains unanswered: Where does the heap manager get the memory from Fundamentally, the heap manager uses the Windows virtual memory manager to allocate memory in large chunks The memory is then massaged into different sized blocks to accommodate the allocation requests of the application When the virtual memory chunks are exhausted, the heap manager allocates yet another large chunk of virtual memory, and the process continues The chunks that the heap manager requests from the virtual memory manager are known as heap segments When a heap segment is first created, the underlying virtual memory is mostly reserved, with only a small portion being committed Whenever the heap manager runs out of committed space in the heap segment, it explicitly commits more memory and divides the newly committed space into blocks as more and more allocations are requested Figure 67 illustrates the basic layout of a heap segment
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