Summary of Key Allocator Policies 15 213 Placement policy The course that gives CMU its Zip Dynamic Memory Allocation II October 21 2008 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 Topics Explicit doubly linked free lists Segregated free lists Garbage collection Review of pointers Memory related perils and pitfalls 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 lecture 16 ppt Coalesce as you scan the free list for malloc Coalesce when the amount of external fragmentation reaches some threshold 15 213 F 08 2 Keeping Track of Free Blocks Method 1 Implicit list using lengths links all blocks Explicit Free Lists Maintain list s of free blocks not all blocks 5 4 6 The next free block could be anywhere 2 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 Method 2 Explicit list among the free blocks using pointers within the free blocks 5 4 6 2 3 C Different free lists for different size classes Forward links A Method 4 Blocks sorted by size not discussed B Note links don t have to be in the same order as the blocks Method 3 Segregated free lists A 4 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 B 4 4 4 6 6 4 C 4 4 4 4 Back links 15 213 F 08 1 Allocating From Explicit Free Lists Freeing With Explicit Free Lists Insertion policy Where in the free list do you put a newly freed block Before 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 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 with splitting 7 After Address ordered policy malloc 15 213 F 08 5 Freeing With a LIFO Policy Case 1 Before 15 213 F 08 6 Freeing With a LIFO Policy Case 2 Before free free Root Root Splice out predecessor block coalesce both memory blocks and insert the new block at the root of the list Insert the freed block at the root of the list After After Root 7 Root 5 15 213 F 08 8 15 213 F 08 2 Freeing With a LIFO Policy Case 3 Before Freeing With a LIFO Policy Case 4 Before free Root Root Splice out successor block coalesce both memory blocks and insert the new block at the root of the list Splice out predecessor and successor blocks coalesce all 3 memory blocks and insert the new block at the root of the list After After Root Root 15 213 F 08 9 15 213 F 08 10 Keeping Track of Free Blocks Explicit List Summary Method 1 Implicit list using lengths links all blocks 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 5 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 Most common use of linked lists is in conjunction with segregated free lists Keep multiple linked lists of different size classes or possibly for different types of objects 5 15 213 F 08 6 2 4 6 2 Method 3 Segregated free list Different free lists for different size classes Method 4 Blocks sorted by size 11 4 Method 2 Explicit list among the free blocks using pointers within the free blocks Does this increase internal frag free 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 3 Seglist Allocator Segregated List Seglist Allocators Given an array of free lists for different size classes Each size class of blocks has its own free list To allocate a block of size n 1 2 3 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 4 5 8 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 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 15 213 F 08 13 15 213 F 08 14 Seglist Allocator cont For More Info on Allocators To free a block D Knuth The Art of Computer Programming Second Edition Addison Wesley 1973 Coalesce and place on appropriate list optional The classic reference on dynamic storage allocation 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 Wilson et al Dynamic Storage Allocation A Survey and Critical Review Proc 1995 Int l Workshop on Memory Management Kinross Scotland Sept 1995 15 15 213 F 08 16 Comprehensive survey Available from CS APP student site csapp cs cmu edu 15 213 F 08 4 Dereferencing Bad Pointers Memory Related Perils and Pitfalls The classic scanf bug Dereferencing bad pointers Reading uninitialized memory Overwriting memory int val Referencing nonexistent variables Freeing blocks multiple times Referencing freed blocks scanf d val Failing to free blocks 15 213 F 08 17 Reading Uninitialized Memory Assuming that heap data is initialized to zero 15 213 F 08 18 Overwriting Memory Allocating the possibly wrong sized object return y Ax int matvec int A int x int y malloc N sizeof int int i j int p p malloc N sizeof int for i 0 i N i p i malloc M sizeof int 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 20 15 213 F 08 5 Overwriting Memory Overwriting Memory Not checking the max string size Off by one error int p char s 8 int i p malloc N sizeof int gets s for i 0 i N i p i malloc M sizeof …
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