15 213 The course that gives CMU its Zip Dynamic Memory Allocation II November 3 2006 Topics class19 ppt Explicit doubly linked free lists Segregated free lists Garbage collection Review of pointers Memory related perils and pitfalls Keeping Track of Free Blocks Method 1 Implicit list using lengths links all blocks 5 4 6 2 Method 2 Explicit list among the free blocks using pointers within the free blocks 5 4 6 2 Method 3 Segregated free lists Different free lists for different size classes Method 4 Blocks sorted by size not discussed 2 Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key 15 213 F 06 Explicit Free Lists A B C Use data space for link pointers Typically doubly linked Still need boundary tags for coalescing Forward links A 4 4 4 4 6 6 4 C 3 4 4 B 4 Back links It is important to realize that links are not necessarily in the same order as the blocks 15 213 F 06 Allocating From Explicit Free Lists Before with splitting After malloc 4 15 213 F 06 Freeing With Explicit Free Lists Insertion policy Where in the free list do you put a newly freed block LIFO last in first out policy Insert freed block at the beginning of the free list Pro simple and constant time Con studies suggest fragmentation is worse than address ordered Address ordered policy Insert freed blocks so that free list blocks are always in address order i e addr pred addr curr addr succ Con requires search Pro studies suggest fragmentation is lower than LIFO 5 15 213 F 06 Freeing With a LIFO Policy Case 1 Before free Root After Root Insert the freed block at the root of the list 6 15 213 F 06 Freeing With a LIFO Policy Case 2 Before free Root After Root Splice out predecessor block coalesce both memory blocks and insert the new block at the root of the list 7 15 213 F 06 Freeing With a LIFO Policy Case 3 Before free Root After Root Splice out successor block coalesce both memory blocks and insert the new block at the root of the list 8 15 213 F 06 Freeing With a LIFO Policy Case 4 Before free Root After Root Splice out predecessor and successor blocks coalesce all 3 memory blocks and insert the new block at the root of the list 9 15 213 F 06 Explicit List Summary Comparison to implicit list Allocate is linear time in number of free blocks instead of total blocks much faster allocates when most of the memory is full Slightly more complicated allocate and free since needs to splice blocks in and out of the list Some extra space for the links 2 extra words needed for each block Does this increase internal frag Main use of linked lists is in conjunction with segregated free lists 10 Keep multiple linked lists of different size classes or possibly for different types of objects 15 213 F 06 Keeping Track of Free Blocks Method 1 Implicit list using lengths links all blocks 5 4 6 2 Method 2 Explicit list among the free blocks using pointers within the free blocks 5 4 6 2 Method 3 Segregated free list Different free lists for different size classes Method 4 Blocks sorted by size 11 Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key 15 213 F 06 Segregated List seglist Allocators Each size class of blocks has its own free list 1 2 3 4 5 8 9 inf 12 Often have separate size class for every small size 2 3 4 For larger sizes typically have a size class for each power of 2 15 213 F 06 Seglist Allocator Given an array of free lists each one for some size class To allocate a block of size n Search appropriate free list for block of size m n If an appropriate block is found Split block and place fragment on appropriate list optional If no block is found try next larger class Repeat until block is found If no block is found 13 Request additional heap memory from OS using sbrk function Allocate block of n bytes from this new memory Place remainder as a single free block in largest size class 15 213 F 06 Seglist Allocator cont To free a block Coalesce and place on appropriate list optional Advantages of seglist allocators Higher throughput i e log time for power of two size classes Better memory utilization First fit search of segregated free list approximates a best fit search of entire heap Extreme case Giving each block its own size class is equivalent to best fit 14 15 213 F 06 For More Info on Allocators D Knuth The Art of Computer Programming Second Edition Addison Wesley 1973 The classic reference on dynamic storage allocation Wilson et al Dynamic Storage Allocation A Survey and Critical Review Proc 1995 Int l Workshop on Memory Management Kinross Scotland Sept 1995 15 Comprehensive survey Available from CS APP student site csapp cs cmu edu 15 213 F 06 Implicit Memory Management Garbage Collection Garbage collection automatic reclamation of heapallocated storage application never has to free void foo int p malloc 128 return p block is now garbage Common in functional languages scripting languages and modern object oriented languages Lisp ML Java Perl Mathematica Variants conservative garbage collectors exist for C and C 16 However cannot necessarily collect all garbage 15 213 F 06 Garbage Collection How does the memory manager know when memory can be freed In general we cannot know what is going to be used in the future since it depends on conditionals But we can tell that certain blocks cannot be used if there are no pointers to them Need to make certain assumptions about pointers 17 Memory manager can distinguish pointers from nonpointers All pointers point to the start of a block Cannot hide pointers e g by coercing them to an int and then back again 15 213 F 06 Classical GC Algorithms Mark and sweep collection McCarthy 1960 Does not move blocks unless you also compact Reference counting Collins 1960 Does not move blocks not discussed Copying collection Minsky 1963 Moves blocks not discussed Generational Collectors Lieberman and Hewitt 1983 Collects based on lifetimes For more information see Jones and Lin Garbage Collection Algorithms for Automatic Dynamic Memory John Wiley Sons 1996 18 15 213 F 06 Memory as a Graph We view memory as a directed graph Each block is a node in the graph Each pointer is an edge in the graph Locations not in the heap that contain pointers into the heap are called root nodes e g registers locations on the stack global variables Root nodes Heap nodes reachable Not reachable garbage A node block is reachable if there is a path from any root to that node Non reachable nodes are
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