15 213 The course that gives CMU its Zip Dynamic Memory Allocation II October 21 2008 Topics Explicit doubly linked free lists Segregated free lists Garbage collection Review of pointers Memory related perils and pitfalls lecture 16 ppt Summary of Key Allocator Policies Placement policy First fit next fit best fit etc Trades off lower throughput for less fragmentation Interesting observation segregated free lists next lecture approximate a best fit placement policy without having to search entire free list Splitting policy When do we go ahead and split free blocks How much internal fragmentation are we willing to tolerate Coalescing policy Immediate coalescing coalesce each time free is called Deferred coalescing try to improve performance of free by deferring coalescing until needed e g Coalesce as you scan the free list for malloc Coalesce when the amount of external fragmentation reaches some threshold 2 15 213 F 08 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 3 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 08 Explicit Free Lists Maintain list s of free blocks not all blocks The next free block could be anywhere So we need to store pointers not just sizes Still need boundary tags for coalescing Luckily we track only free blocks so we can use payload area A B C Note links don t have to be in the same order as the blocks Forward links A 4 4 4 4 6 6 4 C 4 4 4 B 4 Back links 15 213 F 08 Allocating From Explicit Free Lists Before with splitting 7 After malloc 5 15 213 F 08 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 6 15 213 F 08 Freeing With a LIFO Policy Case 1 Before free Root Insert the freed block at the root of the list After Root 7 5 15 213 F 08 Freeing With a LIFO Policy Case 2 Before free Root Splice out predecessor block coalesce both memory blocks and insert the new block at the root of the list After Root 8 15 213 F 08 Freeing With a LIFO Policy Case 3 Before free Root Splice out successor block coalesce both memory blocks and insert the new block at the root of the list After Root 9 15 213 F 08 Freeing With a LIFO Policy Case 4 Before free Root Splice out predecessor and successor blocks coalesce all 3 memory blocks and insert the new block at the root of the list After Root 10 15 213 F 08 Explicit List Summary Comparison to implicit list Allocate is linear time in of free blocks instead of total blocks Allocations much faster 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 free block Does this increase internal frag Most common use of linked lists is in conjunction with segregated free lists 11 Keep multiple linked lists of different size classes or possibly for different types of objects 15 213 F 08 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 12 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 08 Segregated List Seglist Allocators Each size class of blocks has its own free list 1 2 3 4 5 8 9 inf Often have separate size class for each small size 2 3 4 For larger sizes typically have a size class for each power of 2 13 15 213 F 08 Seglist Allocator Given an array of free lists for different size classes 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 Request additional heap memory from OS using sbrk Allocate block of n bytes from this new memory Place remainder as a single free block in largest size class 14 15 213 F 08 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 15 15 213 F 08 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 16 Comprehensive survey Available from CS APP student site csapp cs cmu edu 15 213 F 08 Memory Related Perils and Pitfalls Dereferencing bad pointers Reading uninitialized memory Overwriting memory Referencing nonexistent variables Freeing blocks multiple times Referencing freed blocks Failing to free blocks 17 15 213 F 08 Dereferencing Bad Pointers The classic scanf bug int val scanf d val 18 15 213 F 08 Reading Uninitialized Memory Assuming that heap data is initialized to zero return y Ax int matvec int A int x int y malloc N sizeof int int i j for i 0 i N i for j 0 j N j y i A i j x j return y 19 15 213 F 08 Overwriting Memory Allocating the possibly wrong sized object int p p malloc N sizeof int for i 0 i N i p i malloc M sizeof int 20 15 213 F 08 Overwriting Memory Off by one error int p p malloc N sizeof int for i 0 i N i p i malloc M sizeof int 21 15 213 F 08 Overwriting Memory Not checking the max string size char s 8 int i gets …
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