Page 1Dynamic Memory Allocation IOctober 24, 2007Dynamic Memory Allocation IOctober 24, 2007Topics Simple explicit allocatorsz Data structuresz Mechanismsz Policiesclass16.ppt15-213“The course that gives CMU its Zip!”–2–15-213, F’07Harsh RealityHarsh RealityMemory MattersMemory is not unbounded It must be allocated and managed Many applications are memory dominatedz Especially those based on complex, graph algorithmsMemory referencing bugs especially pernicious Effects are distant in both time and spaceMemory 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–3–15-213, F’07Dynamic Memory AllocationDynamic Memory AllocationExplicit 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 spacez E.g. garbage collection in Java, ML or LispAllocation In both cases the memory allocator provides an abstraction of memory as a set of blocks Doles out free memory blocks to applicationWill discuss simple explicit memory allocation todayApplicationDynamic Memory AllocatorHeap Memory–4–15-213, F’07Process Memory ImageProcess Memory Imagekernel virtual memoryMemory mapped region forshared librariesrun-time heap (via malloc)program text (.text)initialized data (.data)uninitialized data (.bss)stack0%espmemory invisibleto user codethe “brk”ptrAllocators requestadditional heap memoryfrom the operating system using the sbrkfunction.Page 2–5–15-213, F’07Malloc PackageMalloc 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.–6–15-213, F’07Malloc ExampleMalloc Examplevoid 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 */}–7–15-213, F’07AssumptionsAssumptionsAssumptions made in this lecture Memory is word addressed (each word can hold a pointer)Allocated block(4 words)Free block(3 words)Free wordAllocated word–8–15-213, F’07Allocation ExamplesAllocation Examplesp1 = malloc(4)p2 = malloc(5)p3 = malloc(6)free(p2)p4 = malloc(2)Page 3–9–15-213, F’07ConstraintsConstraintsApplications: Can issue arbitrary sequence of allocation and free requests Free requests must correspond to an allocated blockAllocators Can’t control number or size of allocated blocks Must respond immediately to all allocation requestszi.e., can’t reorder or buffer requests Must allocate blocks from free memoryzi.e., can only place allocated blocks in free memory Must align blocks so they satisfy all alignment requirementsz8 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 allocatedzi.e., compaction is not allowed–10–15-213, F’07Performance Goals: ThroughputPerformance Goals: ThroughputGiven some sequence of malloc and free requests: R0, R1, ..., Rk, ... , Rn-1Want to maximize throughput and peak memory utilization. These goals are often conflictingThroughput: 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.–11–15-213, F’07Performance Goals: Peak Memory UtilizationPerformance Goals: Peak Memory UtilizationGiven some sequence of malloc and free requests: R0, R1, ..., Rk, ... , Rn-1Def: Aggregate payload Pk: malloc(p) results in a block with a payload of p bytes. After request Rkhas completed, the aggregate payload Pkis the sum of currently allocated payloads.Def: Current heap size is denoted by Hk Assume that Hkis monotonically nondecreasingDef: Peak memory utilization: After k requests, peak memory utilization is:z Uk= ( maxi<kPi ) / Hk–12–15-213, F’07Internal FragmentationInternal FragmentationPoor memory utilization caused by fragmentation. Comes in two forms: internal and external fragmentationInternal fragmentation For some block, internal fragmentation is the difference between the block size and the payload size. 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.payloadInternal fragmentationblockInternal fragmentationPage 4–13–15-213, F’07External FragmentationExternal Fragmentationp1 = malloc(4)p2 = malloc(5)p3 = malloc(6)free(p2)p4 = malloc(6)oops!Occurs when there is enough aggregate heap memory, but no singlefree block is large enoughExternal fragmentation depends on the pattern of future requests, andthus is difficult to measure. –14–15-213, F’07Implementation IssuesImplementation Issuesz 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?–15–15-213, F’07Knowing How Much to FreeKnowing How Much to FreeStandard method Keep the length of a block in the word preceding the block.zThis word is often called the header field or header Requires an extra word for every allocated blockfree(p0)p0 = malloc(4) p0Block size data5–16–15-213, F’07Keeping Track of Free BlocksKeeping Track of Free BlocksMethod 1: Implicit list using lengths -- links all blocksMethod 2: Explicit list among the free
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