Carnegie Mellon Dynamic Memory Allocation Advanced Concepts 15 213 Introduction to Computer Systems 18th Lecture Oct 26 2010 Instructors Randy Bryant and Dave O Hallaron 1 Carnegie Mellon Today Explicit free lists Segregated free lists Garbage collection Memory related perils and pitfalls 2 Carnegie Mellon Keeping Track of Free Blocks Method 1 Implicit free list using length links all blocks 5 6 2 Method 2 Explicit free list among the free blocks using pointers 5 4 4 6 2 Method 3 Segregated free list Different free lists for different size classes Method 4 Blocks sorted by size Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key 3 Carnegie Mellon Explicit Free Lists Allocated as before Size a Free Size a Next Prev Payload and padding Size a Size a Maintain list s of free blocks not all blocks The next free block could be anywhere So we need to store forward back pointers not just sizes Still need boundary tags for coalescing Luckily we track only free blocks so we can use payload area 4 Carnegie Mellon Explicit Free Lists Logically A B C Physically blocks can be in any order Forward next links A 4 B 4 4 4 6 6 4 C 4 4 4 Back prev links 5 Carnegie Mellon Allocating From Explicit Free Lists conceptual graphic Before After with splitting malloc 6 Carnegie Mellon 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 addr prev addr curr addr next Con requires search Pro studies suggest fragmentation is lower than LIFO 7 Carnegie Mellon Freeing With a LIFO Policy Case 1 conceptual graphic Before free Root Insert the freed block at the root of the list After Root 8 Carnegie Mellon Freeing With a LIFO Policy Case 2 conceptual graphic Before free Root Splice out predecessor block coalesce both memory blocks and insert the new block at the root of the list After Root 9 Carnegie Mellon Freeing With a LIFO Policy Case 3 conceptual graphic Before free Root Splice out successor block coalesce both memory blocks and insert the new block at the root of the list After Root 10 Carnegie Mellon Freeing With a LIFO Policy Case 4 conceptual graphic 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 11 Carnegie Mellon Explicit List Summary Comparison to implicit list Allocate is linear time in number of free blocks instead of all blocks 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 block Does this increase internal fragmentation 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 12 Carnegie Mellon Keeping Track of Free Blocks Method 1 Implicit list using length links all blocks 5 6 2 Method 2 Explicit list among the free blocks using pointers 5 4 4 6 2 Method 3 Segregated free list Different free lists for different size classes Method 4 Blocks sorted by size Can use a balanced tree e g Red Black tree with pointers within each free block and the length used as a key 13 Carnegie Mellon Today Explicit free lists Segregated free lists Garbage collection Memory related perils and pitfalls 14 Carnegie Mellon 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 classes for each small size For larger sizes One class for each two power size 15 Carnegie Mellon 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 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 16 Carnegie Mellon Seglist Allocator cont To free a block Coalesce and place on appropriate list optional Advantages of seglist allocators Higher throughput 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 17 Carnegie Mellon More Info on Allocators D Knuth The Art of Computer Programming 2nd 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 Comprehensive survey Available from CS APP student site csapp cs cmu edu 18 Carnegie Mellon Today Explicit free lists Segregated free lists Garbage collection Memory related perils and pitfalls 19 Carnegie Mellon Implicit Memory Management Garbage Collection Garbage collection automatic reclamation of heap allocated 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 However cannot necessarily collect all garbage 20 Carnegie Mellon 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 Must make certain assumptions about pointers Memory manager can distinguish pointers from non pointers All pointers point to the start of a block Cannot hide pointers e g by coercing them to an int and then back again 21 Carnegie Mellon 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
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