Harsh Reality 15 213 Memory Matters The course that gives CMU its Zip Memory is not unbounded Dynamic Memory Allocation I Apr 1 2003 It must be allocated and managed Many applications are memory dominated z Especially those based on complex graph algorithms Memory referencing bugs especially pernicious Topics Simple explicit allocators Effects are distant in both time and space Memory performance is not uniform z Data structures z Mechanisms z Policies 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 03 2 class20 ppt Dynamic Memory Allocation Process Memory Image Application kernel virtual memory Dynamic Memory Allocator memory invisible to user code stack esp Heap Memory Explicit vs Implicit Memory Allocator Explicit application allocates and frees space Implicit application allocates but does not free space Memory mapped region for shared libraries Allocators request additional heap memory from the operating system using the sbrk function z E g malloc and free in C z E g garbage collection in Java ML or Lisp the brk ptr run time heap via malloc Allocation In both cases the memory allocator provides an abstraction of memory as a set of blocks Doles out free memory blocks to application uninitialized data bss initialized data data program text text Will discuss simple explicit memory allocation today 3 15 213 F 03 4 0 15 213 F 03 Malloc Example Malloc Package void foo int n int m int i p include stdlib h void malloc size t size 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 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 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 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 print new array for i 0 i n m i printf d n p i free p return p to available memory pool 15 213 F 03 5 Assumptions 6 Allocation Examples p1 malloc 4 Assumptions made in this lecture 15 213 F 03 Memory is word addressed each word can hold a pointer p2 malloc 5 Allocated block 4 words Free block 3 words p3 malloc 6 Free word Allocated word free p2 p4 malloc 2 7 15 213 F 03 8 15 213 F 03 Constraints Goals of Good malloc free Applications Can issue arbitrary sequence of allocation and free requests Free requests must correspond to an allocated block Primary goals Good time performance for malloc and free z Ideally should take constant time not always possible Allocators z Should certainly not take linear time in the number of blocks Can t control number or size of allocated blocks Must respond immediately to all allocation requests Good space utilization z User allocated structures should be large fraction of the heap z Want to minimize fragmentation z i e can t reorder or buffer requests Must allocate blocks from free memory Some other goals z i e can only place allocated blocks in free memory Good locality properties Must align blocks so they satisfy all alignment requirements z Structures allocated close in time should be close in space z 8 byte alignment for GNU malloc libc malloc on Linux boxes z Similar objects should be allocated close in space Can only manipulate and modify free memory Can t move the allocated blocks once they are allocated Robust z Can check that free p1 is on a valid allocated object p1 z Can check that memory references are to allocated space z i e compaction is not allowed 15 213 F 03 9 Performance Goals Throughput 15 213 F 03 10 Performance Goals Peak Memory Utilization Given some sequence of malloc and free requests R0 R1 Rk Rn 1 Given some sequence of malloc and free requests Want to maximize throughput and peak memory utilization Def Aggregate payload Pk These goals are often conflicting Throughput 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 Number of completed requests per unit time Example Assume that Hk is monotonically nondecreasing Def Peak memory utilization z 5 000 malloc calls and 5 000 free calls in 10 seconds z Throughput is 1 000 operations second 11 R0 R1 Rk Rn 1 After k requests peak memory utilization is z Uk maxi k Pi Hk 15 213 F 03 12 15 213 F 03 Internal Fragmentation External Fragmentation Occurs when there is enough aggregate heap memory but no single free block is large enough Poor memory utilization caused by fragmentation Comes in two forms internal and external fragmentation p1 malloc 4 Internal fragmentation For some block internal fragmentation is the difference between the block size and the payload size p2 malloc 5 block Internal fragmentation p3 malloc 6 Internal fragmentation payload free p2 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 03 13 Implementation Issues p4 malloc 6 oops External fragmentation depends on the pattern of future requests and thus is difficult to measure 15 213 F 03 14 Knowing How Much to Free z How do we know how much memory to free just given a pointer Standard method Keep the length of a block in the word preceding the block Requires an extra word for every allocated block z This word is often called the header field or header 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 p0 malloc 4 p0 5 z How do we reinsert freed block p0 free p0 Block size data free p0 p1 malloc 1 15 15 213 F 03 16 15 213 F 03 Keeping Track of Free Blocks Method 1 Implicit List Method 1 Implicit list using lengths links all blocks Need to identify whether each block is free or allocated 5 4 6 2 Method 2 Explicit list among the free blocks using pointers within the free blocks Can use extra bit Bit can be put in the
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