Dynamic Memory Allocation I November 1, 2006Harsh RealityDynamic Memory AllocationProcess Memory ImageMalloc PackageMalloc ExampleAssumptionsAllocation ExamplesConstraintsPerformance Goals: ThroughputPerformance Goals: Peak Memory UtilizationInternal FragmentationExternal FragmentationImplementation IssuesKnowing How Much to FreeKeeping Track of Free BlocksMethod 1: Implicit ListImplicit List: Finding a Free BlockBitfieldsImplicit List: Allocating in Free BlockImplicit List: Freeing a BlockImplicit List: CoalescingImplicit List: Bidirectional CoalescingConstant Time CoalescingConstant Time Coalescing (Case 1)Constant Time Coalescing (Case 2)Constant Time Coalescing (Case 3)Constant Time Coalescing (Case 4)Summary of Key Allocator PoliciesImplicit Lists: SummaryDynamic Memory Allocation INovember 1, 2006Dynamic Memory Allocation INovember 1, 2006TopicsSimple explicit allocatorsData structuresMechanismsPoliciesclass18.ppt15-213“The course that gives CMU its Zip!”– 2 –15-213, F’06Harsh RealityHarsh RealityMemory MattersMemory is not unboundedIt must be allocated and managedMany applications are memory dominatedEspecially those based on complex, graph algorithmsMemory referencing bugs especially perniciousEffects are distant in both time and spaceMemory performance is not uniformCache and virtual memory effects can greatly affect program performanceAdapting program to characteristics of memory system can lead to major speed improvements– 3 –15-213, F’06Dynamic Memory AllocationDynamic Memory AllocationExplicit vs. Implicit Memory AllocatorExplicit: application allocates and frees space E.g., malloc and free in CImplicit: application allocates, but does not free spaceE.g. garbage collection in Java, ML or LispAllocationIn both cases the memory allocator provides an abstraction of memory as a set of blocksDoles out free memory blocks to applicationWill discuss simple explicit memory allocation todayApplicationDynamic Memory AllocatorHeap Memory– 4 –15-213, F’06Process 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 sbrk function.– 5 –15-213, F’06Malloc PackageMalloc Package#include <stdlib.h>void *malloc(size_t size)If successful:Returns a pointer to a memory block of at least size bytes, (typically) aligned to 8-byte boundary.If size == 0, returns NULLIf unsuccessful: returns NULL (0) and sets errno.void free(void *p)Returns the block pointed at by p to pool of available memoryp 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’06Malloc 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’06AssumptionsAssumptionsAssumptions made in this lectureMemory is word addressed (each word can hold a pointer)Allocated block(4 words)Free block(3 words)Free wordAllocated word– 8 –15-213, F’06Allocation ExamplesAllocation Examplesp1 = malloc(4)p2 = malloc(5)p3 = malloc(6)free(p2)p4 = malloc(2)– 9 –15-213, F’06ConstraintsConstraintsApplications:Can issue arbitrary sequence of allocation and free requestsFree requests must correspond to an allocated blockAllocatorsCan’t control number or size of allocated blocksMust respond immediately to all allocation requestsi.e., can’t reorder or buffer requestsMust allocate blocks from free memoryi.e., can only place allocated blocks in free memoryMust align blocks so they satisfy all alignment requirements8 byte alignment for GNU malloc (libc malloc) on Linux boxesCan only manipulate and modify free memoryCan’t move the allocated blocks once they are allocatedi.e., compaction is not allowed– 10 –15-213, F’06Performance 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 timeExample:5,000 malloc calls and 5,000 free calls in 10 seconds Throughput is 1,000 operations/second.– 11 –15-213, F’06Performance 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 Rk has completed, the aggregate payload Pk is the sum of currently allocated payloads.Def: Current heap size is denoted by HkAssume that Hk is monotonically nondecreasingDef: Peak memory utilization: After k requests, peak memory utilization is:Uk = ( maxi<k Pi ) / Hk– 12 –15-213, F’06Internal FragmentationInternal FragmentationPoor memory utilization caused by fragmentation.Comes in two forms: internal and external fragmentationInternal fragmentationFor 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 fragmentation– 13 –15-213, F’06External 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’06Implementation
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