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| Let's start by describing how the data on a freelist is laid out in memory. |
| This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes): |
| </p><pre class="programlisting"> |
| +----------------+ |
| | next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here) |
| | | | |
| | | | |
| | | | |
| +----------------+ | |
| | thread_id = 3 | | |
| | | | |
| | | | |
| | | | |
| +----------------+ | |
| | DATA | | (A pointer to here is what is returned to the |
| | | | the application when needed) |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
| +----------------+ | |
| +----------------+ | |
| | next* |<-+ (If next == NULL it's the last one on the list) |
| | | |
| | | |
| | | |
| +----------------+ |
| | thread_id = 3 | |
| | | |
| | | |
| | | |
| +----------------+ |
| | DATA | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| +----------------+ |
| </pre><p> |
| With this in mind we simplify things a bit for a while and say that there is |
| only one thread (a ST application). In this case all operations are made to |
| what is referred to as the global pool - thread id 0 (No thread may be |
| assigned this id since they span from 1 to _S_max_threads in a MT application). |
| </p><p> |
| When the application requests memory (calling allocate()) we first look at the |
| requested size and if this is > _S_max_bytes we call new() directly and return. |
| </p><p> |
| If the requested size is within limits we start by finding out from which |
| bin we should serve this request by looking in _S_binmap. |
| </p><p> |
| A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of |
| this size on the freelist (0). If this is not NULL - fine, just remove the |
| block that _S_bin[ bin ].first[ 0 ] points to from the list, |
| update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data. |
| </p><p> |
| If the freelist is empty (the pointer is NULL) we must get memory from the |
| system and build us a freelist within this memory. All requests for new memory |
| is made in chunks of _S_chunk_size. Knowing the size of a block_record and |
| the bytes that this bin stores we then calculate how many blocks we can create |
| within this chunk, build the list, remove the first block, update the pointer |
| (_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data. |
| </p><p> |
| Deallocation is equally simple; the pointer is casted back to a block_record |
| pointer, lookup which bin to use based on the size, add the block to the front |
| of the global freelist and update the pointer as needed |
| (_S_bin[ bin ].first[ 0 ]). |
| </p><p> |
| The decision to add deallocated blocks to the front of the freelist was made |
| after a set of performance measurements that showed that this is roughly 10% |
| faster than maintaining a set of "last pointers" as well. |
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