15 213 Dynamic Memory Allocation I November 5 2007 Topics Simple explicit allocators z Data structures z Mechanisms z Policies class18 ppt 15 213 F 07 Harsh Reality Memory Matters Memory is not unbounded It must be allocated and managed Many applications are memory dominated z 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 2 Cache and virtual memory effects can greatly affect program performance Adapting program to characteristics of memory system can lead to major speed improvements 15 213 F 07 Dynamic Memory Allocation Application Dynamic Memory Allocator Heap Memory Explicit vs Implicit Memory Allocator Explicit application allocates and frees space z E g malloc and free in C Implicit application allocates but does not free space z 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 3 15 213 F 07 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 4 0 15 213 F 07 Malloc Package include stdlib h void malloc size t size If successful z Returns a pointer to a memory block of at least size bytes typically aligned to 8 byte boundary z If size 0 returns NULL If unsuccessful returns NULL 0 and sets errno 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 pointer to new block Contents of new block unchanged up to min of old and new size 5 15 213 F 07 Malloc Example void foo int n int m int i p allocate a block of n ints p int malloc n sizeof int if p 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 6 15 213 F 07 Assumptions Assumptions made in this lecture Memory is word addressed each word can hold a pointer Allocated block 4 words 7 Free block 3 words Free word Allocated word 15 213 F 07 Allocation Examples p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 2 8 15 213 F 07 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 z i e can t reorder or buffer requests Must allocate blocks from free memory z i e can only place allocated blocks in free memory Must align blocks so they satisfy all alignment requirements z 8 byte alignment for GNU malloc libc malloc on Linux boxes Can only manipulate and modify free memory Can t move the allocated blocks once they are allocated z i e compaction is not allowed 9 15 213 F 07 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 z 5 000 malloc calls and 5 000 free calls in 10 seconds z Throughput is 1 000 operations second 10 15 213 F 07 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 Assume that Hk is monotonically nondecreasing Def Peak memory utilization After k requests peak memory utilization is z Uk maxi k Pi Hk 11 15 213 F 07 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 12 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 15 213 F 07 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 13 15 213 F 07 Implementation Issues z How do we know how much memory to free just given a pointer z How do we keep track of the free blocks z What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in z How do we pick a block to use for allocation many might fit z How do we reinsert freed block 14 15 213 F 07 Knowing How Much to Free Standard method Keep the length of a block in the word preceding the block z This word is often called the header field or header Requires an extra word for every allocated block p0 p0 malloc 4 5 free p0 15 Block size data 15 213 F 07 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 16 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 07 Method 1 Implicit List Need to identify whether each block is free or allocated Can use extra bit Bit can be put in the same word as the size if block sizes are always multiples of two mask out low order bit when reading size 1 word size Format of allocated and free blocks a a 1 allocated block a 0 free block size block size payload payload application data allocated blocks only optional padding 17 15 213 F 07 Implicit List Finding a Free Block First fit Search list from beginning choose first free block …
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