Section9

When we allocate memory to a process “malloc”, the Pool Manager takes a free memory block and attaches it to a process. The Pool Manager maintains a table of contiguous memory blocks. When a process frees a memory block, the Pool Manager tries to concatenate it with its neighbors. Despite of this concatenation, fragmentation is unavoidable. An extremely fragmented memory can lead to “malloc” errors “%SYS-2-MALLOCFAIL”. Indeed, the memory available can be sufficient, but not enough contiguous free blocks available to allow a requested “malloc”. In the last figure, there are only small non-contiguous freed blocks: Now, if a process wants to allocate a larger memory area, it will not be able to do it and it’ll have a “MALLOCFAIL” error. The Chunk Manager will try to avoid this kind of situation by allocating the memory to processes cleverly. The Chunk Manager is responsible of the Chunks Allocation. A Chunk contains a finished number of memory blocks of equal size. If we use every memory blocks of a Chunk, the Chunk Manager will allocate a new memory area (Sibling). If no more blocks of this “Sibling” are used, it is freed “Freed/Trimmed”. A process now has a larger allocated memory area split into smaller memory blocks. When a process frees a memory block, it does in its Chunk. There is no more fragmentation due to different processes freeing memory.

#show chunk
Chunk Manager:  407660 chunks created, 406281 chunks destroyed  9349 siblings created, 406279 siblings trimmed
Chunk          element  Block   Maximum  Element  Element
Total Cfgsize   Ohead    size   element    inuse   freed  Ohead    Name
16               4       940       33        2      31     360   String-DB owne 0x654C2408
16               4       940       33        0      33     360   String-DB cont 0x654C27B4
312              16    65588      197       38     159    4072   Extended ACL e 0x654C3484
96               16    20052      171        9     162    3584   ACL Header 0x654D34B8
8536             0     65588        7        1       6    5784   Parseinfo Bloc 0x654DA14C
16               0       456       15        1      14     164   tokenQ node 0x654EA180
20               0       456       13       13       0     144   Chain Cache No 0x654EA348
20               0       456       13        6       7     144   (sibling) 0x66BD0E50
20               0       460       13       13       0     148   (sibling) 0x66F33EE0

Within a Chunk, the memory blocks have a header (or not, size = 0). The header size per chunk is fixed (0, 4, 16, 20, 24). Example: ACL Header is a Chunk of 171 elements max, with 96 elements since creation. Each block have a header of 16Bytes and weight 20,052Bytes. Actually there are only 9 memory blocks in use within this Chunk. Next post: The Buffers.