Harsh Reality 15 213 Memory Matters The course that gives CMU its Zip Memory is not unbounded Dynamic Memory Allocation I October 24 2007 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 Cache and virtual memory effects can greatly affect program performance Adapting program to characteristics of memory system can lead to major speed improvements z Policies 15 213 F 07 2 class16 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 07 4 Page 1 0 15 213 F 07 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 6 15 213 F 07 5 Assumptions Allocation Examples p1 malloc 4 Assumptions made in this lecture 15 213 F 07 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 07 8 Page 2 15 213 F 07 Constraints Performance Goals Throughput Applications Given some sequence of malloc and free requests Can issue arbitrary sequence of allocation and free requests Free requests must correspond to an allocated block Allocators R0 R1 Rk Rn 1 Want to maximize throughput and peak memory utilization Can t control number or size of allocated blocks Must respond immediately to all allocation requests These goals are often conflicting z i e can t reorder or buffer requests Must allocate blocks from free memory Must align blocks so they satisfy all alignment requirements Can only manipulate and modify free memory Can t move the allocated blocks once they are allocated z i e can only place allocated blocks in free memory Throughput z 8 byte alignment for GNU malloc libc malloc on Linux boxes z 5 000 malloc calls and 5 000 free calls in 10 seconds z Throughput is 1 000 operations second z i e compaction is not allowed 15 213 F 07 9 Number of completed requests per unit time Example 15 213 F 07 10 Performance Goals Peak Memory Utilization Internal Fragmentation Poor memory utilization caused by fragmentation Given some sequence of malloc and free requests Comes in two forms internal and external fragmentation R0 R1 Rk Rn 1 Internal fragmentation 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 block Internal fragmentation 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 For some block internal fragmentation is the difference between the block size and the payload size 15 213 F 07 12 Page 3 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 Implementation Issues Occurs when there is enough aggregate heap memory but no single free block is large enough z How do we know how much memory to free just given a pointer p1 malloc 4 z How do we keep track of the free blocks p2 malloc 5 z What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in p3 malloc 6 z How do we pick a block to use for allocation many might fit free p2 z How do we reinsert freed block p4 malloc 6 oops External fragmentation depends on the pattern of future requests and thus is difficult to measure 15 213 F 07 13 15 213 F 07 14 Keeping Track of Free Blocks Knowing How Much to Free Standard method Keep the length of a block in the word preceding the block Requires an extra word for every allocated block Method 1 Implicit list using lengths links all blocks z This word is often called the header field or header 5 4 6 2 Method 2 Explicit list among the free blocks using pointers within the free blocks 5 p0 malloc 4 p0 Block size 2 Different free lists for different size classes Method 4 Blocks sorted by size data 15 6 Method 3 Segregated free list 5 free p0 4 15 213 F 07 16 Page 4 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 Implicit List Finding a Free Block Method 1 Implicit List First fit Need to identify whether each block is free or allocated Search list from beginning choose first free block that fits p start while p end not passed end p 1 already allocated p len too small p p p 2 goto next block Can take linear time in total number of blocks allocated and free In practice it can cause splinters at beginning of list Can use extra bit Bit can be put …
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