15 213 The course that gives CMU its Zip Dynamic Memory Allocation II October 30 2003 Topics class20 ppt Explicit doubly linked free lists Segregated free lists Garbage collection Memory related perils and pitfalls Keeping Track of Free Blocks recap 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 03 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 03 Allocating From Explicit Free Lists Before with splitting After malloc 4 15 213 F 03 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 better than LIFO 5 15 213 F 03 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 03 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 03 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 03 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 03 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 fragmentation 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 03 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 03 Segregated Storage Each size class has its own collection of blocks 1 2 3 4 5 8 9 16 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 03 Simple Segregated Storage Separate heap and free list for each size class No splitting To allocate a block of size n If free list for size n is not empty allocate first block on list note list can be implicit or explicit If free list is empty get a new page create new free list from all blocks in page allocate first block on list Constant time To free a block Add to free list If page is empty return the page for use by another size optional Tradeoffs 13 Fast but can fragment badly 15 213 F 03 Segregated Fits 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 To free a block Coalesce and place on appropriate list optional Tradeoffs Faster search than sequential fits i e log time for power of two size classes Controls fragmentation of simple segregated storage Coalescing can increase search times Deferred coalescing can help 14 15 213 F 03 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 03 Implicit Memory Management Garbage Collection Garbage collection automatic reclamation of heapallocated storage application never has to free Invented 1958 by John McCarthy for Lisp 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 03 Useful malloc Related Information Debugging Tools for Dynamic Storage Allocation and Memory Management http www cs colorado edu homes zorn public html MallocDebug html Electric Fence from Bruce Perens http perens com FreeSoftware IBM P Series AIX systems http publib16 boulder ibm com pseries en US aixprggd genprogc mastertoc htm Memory Allocator for Multithreaded programs FYI http www cs utexas edu users emery hoard 17 15 213 F 03 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 18 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 03 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
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