15 213 The course that gives CMU its Zip Dynamic Memory Allocation I Nov 5 2002 Topics n Simple explicit allocators l Data structures l Mechanisms l Policies class21 ppt Harsh Reality Memory Matters Memory is not unbounded n It must be allocated and managed n Many applications are memory dominated l Especially those based on complex graph algorithms Memory referencing bugs especially pernicious n Effects are distant in both time and space Memory performance is not uniform 2 n Cache and virtual memory effects can greatly affect program performance n Adapting program to characteristics of memory system can lead to major speed improvements 15 213 F 02 Dynamic Memory Allocation Application Dynamic Memory Allocator Heap Memory Explicit vs Implicit Memory Allocator n Explicit application allocates and frees space l E g malloc and free in C n Implicit application allocates but does not free space l E g garbage collection in Java ML or Lisp Allocation n n 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 02 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 02 Malloc Package include stdlib h void malloc size t size n If successful l Returns a pointer to a memory block of at least size bytes typically aligned to 8 byte boundary l If size 0 returns NULL n If unsuccessful returns NULL void free void p n n 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 n n 5 Changes size of block p and returns pointer to new block Contents of new block unchanged up to min of old and new size 15 213 F 02 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 6 free p return p to available memory pool 15 213 F 02 Assumptions Assumptions made in this lecture n 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 02 Allocation Examples p1 malloc 4 p2 malloc 5 p3 malloc 6 free p2 p4 malloc 2 8 15 213 F 02 Constraints Applications n Can issue arbitrary sequence of allocation and free requests n Free requests must correspond to an allocated block Allocators n Can t control number or size of allocated blocks n Must respond immediately to all allocation requests l i e can t reorder or buffer requests n Must allocate blocks from free memory l i e can only place allocated blocks in free memory n Must align blocks so they satisfy all alignment requirements l 8 byte alignment for GNU malloc libc malloc on Linux boxes n Can only manipulate and modify free memory n Can t move the allocated blocks once they are allocated l i e compaction is not allowed 9 15 213 F 02 Goals of Good malloc free Primary goals n Good time performance for malloc and free l Ideally should take constant time not always possible l Should certainly not take linear time in the number of blocks n Good space utilization l User allocated structures should be large fraction of the heap l Want to minimize fragmentation Some other goals n Good locality properties l Structures allocated close in time should be close in space l Similar objects should be allocated close in space n Robust l Can check that free p1 is on a valid allocated object p1 l Can check that memory references are to allocated space 10 15 213 F 02 Performance Goals Throughput Given some sequence of malloc and free requests n R0 R1 Rk Rn 1 Want to maximize throughput and peak memory utilization n These goals are often conflicting Throughput n Number of completed requests per unit time n Example l 5 000 malloc calls and 5 000 free calls in 10 seconds l Throughput is 1 000 operations second 11 15 213 F 02 Performance Goals Peak Memory Utilization Given some sequence of malloc and free requests n R0 R1 Rk Rn 1 Def Aggregate payload Pk n n 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 n Assume that Hk is monotonically nondecreasing Def Peak memory utilization n After k requests peak memory utilization is l Uk maxi k Pi Hk 12 15 213 F 02 Internal Fragmentation Poor memory utilization caused by fragmentation n Comes in two forms internal and external fragmentation Internal fragmentation n For some block internal fragmentation is the difference between the block size and the payload size block Internal fragmentation n n 13 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 02 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 14 15 213 F 02 Implementation Issues l How do we know how much memory to free just given a pointer l How do we keep track of the free blocks l What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in l How do we pick a block to use for allocation many might fit l How do we reinsert freed block p0 free p0 15 p1 malloc 1 15 213 F 02 Knowing How Much to Free Standard method n Keep the length of a block in the word preceding the block l This word is often called the header field or header n Requires an extra word for every allocated block p0 malloc 4 p0 5 free p0 16 Block size data 15 213 F 02 Keeping Track of Free Blocks Method 1 Implicit list using lengths links all blocks 5 4 6 2 Method 2 Explicit …
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