15 213 The course that gives CMU its Zip Dynamic Memory Allocation November 6 2001 Topics class21 ppt Simple explicit allocators Data structures Mechanisms Policies Harsh Reality 3 Memory Matters Memory is not unbounded It must be allocated and managed Many applications are memory dominated Especially those based on complex graph algorithms Memory referencing bugs especially pernicious Effects are distant in both time and space Memory performance is not uniform Cache and virtual memory effects can greatly affect program performance Adapting program to characteristics of memory system can lead to major speed improvements class21 ppt 2 CS 213 F 01 Dynamic Memory Allocation Application Dynamic Memory Allocator Heap Memory Explicit vs Implicit Memory Allocator Explicit application allocates and frees space E g malloc and free in C Implicit application allocates but does not free space E g garbage collection in Java ML or Lisp Allocation In both cases the memory allocator provides an abstraction of memory as a set of blocks Doles out free memory blocks to application Will discuss simple explicit memory allocation today class21 ppt 3 CS 213 F 01 Process memory image kernel virtual memory memory invisible to user code stack esp Memory mapped region for shared libraries Allocators request additional heap memory from the operating system using the sbrk function the brk ptr run time heap via malloc uninitialized data bss initialized data data program text text 0 class21 ppt 4 CS 213 F 01 Malloc package include stdlib h void malloc size t size if successful returns a pointer to a memory block of at least size bytes aligned to 8 byte boundary if size 0 returns NULL if unsuccessful returns NULL void free void p returns the block pointed at by p to pool of available memory p must come from a previous call to malloc or realloc void realloc void p size t size changes size of block p and returns ptr to new block contents of new block unchanged up to min of old and new size class21 ppt 5 CS 213 F 01 Malloc example void foo int n int m int i p allocate a block of n ints if p int malloc n sizeof int NULL perror malloc exit 0 for i 0 i n i p i i add m bytes to end of p block if p int realloc p n m sizeof int NULL perror realloc exit 0 for i n i n m i p i i print new array for i 0 i n m i printf d n p i free p return p to available memory pool class21 ppt 6 CS 213 F 01 Assumptions Assumptions made in this lecture memory is word addressed each word can hold a pointer Allocated block 4 words class21 ppt Free block 3 words 7 Free word Allocated word CS 213 F 01 Allocation examples p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 2 class21 ppt 8 CS 213 F 01 Constraints Applications Can issue arbitrary sequence of allocation and free requests Free requests must correspond to an allocated block Allocators Can t control number or size of allocated blocks Must respond immediately to all allocation requests i e can t reorder or buffer requests Must allocate blocks from free memory i e can only place allocated blocks in free memory Must align blocks so they satisfy all alignment requirements usually 8 byte alignment Can only manipulate and modify free memory Can t move the allocated blocks once they are allocated i e compaction is not allowed class21 ppt 9 CS 213 F 01 Goals of good malloc free Primary goals Good time performance for malloc and free Ideally should take constant time not always possible Should certainly not take linear time in the number of blocks Good space utilization User allocated structures should be large fraction of the heap want to minimize fragmentation Some other goals Good locality properties structures allocated close in time should be close in space similar objects should be allocated close in space Robust can check that free p1 is on a valid allocated object p1 can check that memory references are to allocated space class21 ppt 10 CS 213 F 01 Performance goals throughput Given some sequence of malloc and free requests R0 R1 Rk Rn 1 Want to maximize throughput and peak memory utilization These goals are often conflicting Throughput Number of completed requests per unit time Example 5 000 malloc calls and 5 000 free calls in 10 seconds throughput is 1 000 operations second class21 ppt 11 CS 213 F 01 Performance goals peak memory utilization Given some sequence of malloc and free requests R0 R1 Rk Rn 1 Def aggregate payload Pk malloc p results in a block with a payload of p bytes After request Rk has completed the aggregate payload Pk is the sum of currently allocated payloads Def current heap size is denoted by Hk Note that Hk is monotonically nondecreasing Def peak memory utilization After k requests peak memory utilization is Uk maxi k Pi Hk class21 ppt 12 CS 213 F 01 Internal Fragmentation Poor memory utilization caused by fragmentation Comes in two forms internal and external fragmentation Internal fragmentation For some block internal fragmentation is the difference between the block size and the payload size block Internal fragmentation payload Internal fragmentation Caused by overhead of maintaining heap data structures padding for alignment purposes or explicit policy decisions e g not to split the block Depends only on the pattern of previous requests and thus is easy to measure class21 ppt 13 CS 213 F 01 External fragmentation Occurs when there is enough aggregate heap memory but no single free block is large enough p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 6 oops External fragmentation depends on the pattern of future requests and thus is difficult to measure class21 ppt 14 CS 213 F 01 Implementation issues How do we know how much memory to free just given a pointer How do we keep track of the free blocks What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in How do we pick a block to use for allocation many might fit How do we reinsert freed block p0 free p0 p1 malloc 1 class21 ppt 15 CS 213 F 01 Knowing how much to free Standard method keep the length of a structure in the word preceding the structure This word is often called the header field or header requires an extra word for every allocated structure p0 malloc 4 p0 5 free p0 class21 ppt Block size data 16 CS 213 F 01 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
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