dynamic memory allocation

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Dynamic Memory Allocation

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Outline

• Implementation of a simple allocator• Explicit Free List• Segregated Free List• Suggested reading: 10.9, 10.10, 10.11,

10.12, 10.13

Segregate:隔离

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Dynamic Memory Allocation P731

• Explicit vs. Implicit Memory Allocator– Explicit: application allocates and frees space

• E.g., malloc and free in C

– Implicit: application allocates, but does not free space

• E.g. garbage collection in Java, ML or Lisp

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Dynamic Memory Allocation

• Allocation– In both cases the memory allocator provides an

abstraction of memory as a set of blocks– Doles out free memory blocks to application

Doles: 发放

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10.9.1 The malloc and free Functions

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Malloc package P731

• #include <stdlib.h>• void *malloc(size_t size)

– if successful:• returns a pointer to a memory block of at least size bytes, aligned to

8-byte boundary.• if size==0, returns NULL

– if unsuccessful: returns NULL

• 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,calloc or realloc.

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sbrk() Function P732

• #include <unistd.h>• void *sbrk(int incr)

– If successful• It returns the old value of brk

– If unsuccessful• It returns –1• It sets errno to ENOMEM

– If incr is zero• It returns the current value

– incr can be a negative number

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• Assumptions made in this lecture– memory is word addressed (each word can

hold a pointer)

Allocated block(4 words)

Free block(3 words)

Free word

Allocated word

Assumptions

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p1 = malloc(4)

p2 = malloc(5)

p3 = malloc(6)

free(p2)

p4 = malloc(2)

Allocation examples

Figure 10.36 P733

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10.9.2 Why Dynamic Memory Allocation

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1 #include "csapp.h"2 #define MAXN 1521334 int array[MAXN];56 int main()7 {8 int i, n;910 scanf("%d", &n);11 if (n > MAXN)12 app_error("Input file too big");13 for (i = 0; i < n; i++)14 scanf("%d", &array[i]);15 exit(0);16 }

Why Dynamic Memory Allocation P734

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1 #include "csapp.h"23 int main()4 {5 int *array, i, n;67 scanf("%d", &n);8 array = (int *)Malloc(n * sizeof(int));9 for (i = 0; i < n; i++)10 scanf("%d", &array[i]);11 exit(0);12 }

Why Dynamic Memory Allocation P734

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10.9.3 Allocator Requirements and Goals

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Constraints

• Applications:– Can issue arbitrary sequence of allocation and

free requests– Free requests must correspond to an allocated

block

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Constraints

• Allocators– Can’t control number or size of allocated blocks– Must respond immediately to all allocation

requests• i.e., can’t reorder or buffer requests

– Must allocate blocks from free memory• i.e., can only place allocated blocks in free memory

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Constraints

• Allocators– Must align blocks so they satisfy all alignment

requirements• usually 8 byte alignment

– Can only manipulate and modify free memory– Can’t move the allocated blocks once they are

allocated• i.e., compaction is not allowed

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Goals P735

• Given some sequence of malloc and free requests:– R0, R1, ..., Rk, ... , Rn-1

• Want to maximize throughput and peak memory utilization.– These goals are often conflicting

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Performance goals: throughput

• Number of completed requests per unit time

• Example:– 5,00 malloc calls and 5,00 free calls in 1

seconds – throughput is 1,000 operations/second.

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Performance goals: peak memory utilization


• 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.

Aggregate: 合计，累计

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Performance goals: peak memory utilization


• Def: current heap size is denoted by Hk

– Note that Hk is monotonically nondecreasing• Def: peak memory utilization:

– After k requests, peak memory utilization is:• Uk = ( maxi<k Pi ) / Hk

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10.9.4 Fragmentation

Fragmentation: 分成碎片

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Fragmentation

• Poor memory utilization caused by fragmentation– Comes in two forms:

• internal fragmentation• external fragmentation

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Internal Fragmentation

• Internal fragmentation– For some block, internal fragmentation is the

difference between the block size and the payload size

payloadInternal fragmentation

block

Internal fragmentation

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Internal Fragmentation

• Internal fragmentation– Is 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.

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External fragmentation

• Occurs when there is enough aggregate heap memory, but no single

• free block is large enoughp1 = malloc(4)

p2 = malloc(5)

p3 = malloc(6)

free(p2)

p4 = malloc(6)

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External fragmentation

• External fragmentation depends on – the pattern of future requests– and thus is difficult to measure

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10.9.5 Implementation Issues

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Implementation issues

• How do we know how much memory to free just given a pointer?

• How do we keep track of the free blocks?

p1 = malloc(1)

p0

free(p0)

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Implementation issues

• What do we do with the extra space when allocating a structure that is smaller than the free block it is placed in?

• How do we pick a block to use for allocation – many might fit?

• How do we reinsert freed block?Reinsert：重新插入

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Knowing how much to free

• Standard method– keep the length of a structure in the word

preceding the structure• This word is often called the header field or header

– requires an extra word for every allocated structure

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Knowing how much to free

free(p0)

p0 = malloc(4) p0

Block size data

5

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10.9.6 Implicit Free Lists

Implicit：暗示的，绝对的

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Implicit list

• Need to identify whether each block is free or allocated– Can use extra bit– Bit can be put in the same word as the size if

block sizes are always multiples of 8 (mask out low order bit when reading size).

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Implicit list

size

1 word

Format ofallocated andfree blocks

payload

a = 1: allocated block a = 0: free block

size: block size

payload: application data(allocated blocks only)

a

optionalpadding

00

4 4 26

p

Figure 10.38 P738

Figure 10.37 P738

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10.9.7 Placing Allocated Blocks

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Finding a free block

• 1 ） First fit:– Search list from beginning, choose first free

block that fits– Can take linear time in total number of blocks

(allocated and free)– In practice it can cause “splinters” at

beginning of listp = start; while ((p < end) || \\ not passed end (*p & 1) || \\ already allocated (*p <= len) ); \\ too small

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Finding a free block

• 2 ） Next fit:– Like first-fit, but search list from location of

end of previous search– Research suggests that fragmentation is worse

• 3 ） Best fit:– Search the list, choose the free block with the

closest size that fits– Keeps fragments small --- usually helps

fragmentation– Will typically run slower than first-fit

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10.9.8 Splitting Free Blocks

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Allocating in a free block

• Allocating in a free block - splitting– Since allocated space might be smaller than

free space, we might want to split the block

4 4 26

p

4 24 24

Figure 10.39 P740

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10.9.9 Getting Additional Heap Memory

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10.9.10 Coalescing Free Blocks

Coalescing：接合

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Freeing a block

• Simplest implementation:– Only need to clear allocated flag– But can lead to “false fragmentation” – There is enough free space, but the allocator

won’t be able to find it

4 24 2

free(p) p

4 4 2

4

4 2

malloc(5)

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Coalescing

• Join with next and/or previous block if they are free– Coalescing with next block– But how do we coalesce with previous block?

4 24 2

free(p) p

4 4 2

4

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Figure 10.40 P741

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10.9.11 Coalescing with Boundary Tags

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Bidirectional

• Boundary tags [Knuth73]– replicate size/allocated word at bottom of free

blocks– Allows us to traverse the “list” backwards, but

requires extra space– Important and general technique!

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Bidirectional

4 4 4 4 6 46 4

size

1 word

Format ofallocated andfree blocks

payload andpadding

a = 1: allocated block a = 0: free block

size: block size

payload: application data(allocated blocks only)

a

size aboundary tag (footer)

header 00

00

Figure 10.41 P742

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allocated

allocated

allocated

free

free

allocated

free

free

block beingfreed

Case 1 Case 2 Case 3 Case 4

Constant time coalescing

Figure 10.42 P743

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m1 1

m1 1n 1

n 1m2 1

m2 1

m1 1

m1 1n 0

n 0m2 1

m2 1

Constant time coalescing (case 1)

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m1 1

m1 1n+m2 0

n+m2 0

m1 1

m1 1n 1

n 1m2 0

m2 0


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m1 0

m1 0n 1

n 1m2 1

m2 1

n+m1 0

n+m1 0m2 1

m2 1


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m1 0

m1 0n 1

n 1m2 0

m2 0

n+m1+m2 0

n+m1+m2 0


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10.9.12 Putting it Together: Implementing a Simple Allocator

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1 int mm_init(void);2 void *mm_malloc(size_t size);3 void mm_free(void *bp);

Implementing a Simple Allocator

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Data Structure

Figure 10.44 P745

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1 #include "csapp.h"23 /* private global variables */4 static void *mem_start_brk; /* points to first byte of the heap */5 static void *mem_brk; /* points to last byte of the heap */6 static void *mem_max_addr; /* max virtual address for the heap */78 /*9 * mem_init - initializes the memory system model10 */11 void mem_init(int size)12 {13 mem_start_brk = (void *)Malloc(size); /* models available VM

*/14 mem_brk = mem_start_brk; /* heap is initially

empty */15 mem_max_addr = mem_start_brk + size; /* max VM address for

heap */16 }17

Initialize Figure 10.43 P745

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18 /*19 * mem_sbrk - simple model of the the sbrk function.

Extends the heap20 * by incr bytes and returns the start address of the

new area. In21 * this model, the heap cannot be shrunk.22 */23 void *mem_sbrk(int incr)24 {25 void *old_brk = mem_brk;2627 if ( (incr < 0) || ((mem_brk + incr) > mem_max_addr)) {28 errno = ENOMEM;29 return (void *)-1;30 }31 mem_brk += incr;32 return old_brk;33 }

Initialize

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1 /* Basic constants and macros */2 #define WSIZE 4 /* word size (bytes) */3 #define DSIZE 8 /* doubleword size (bytes) */4 #define CHUNKSIZE (1<<12) /* initial heap size (bytes) */5 #define OVERHEAD 8 /* overhead of header and footer (bytes)

*/67 #define MAX(x, y) ((x) > (y)? (x) : (y))89 /* Pack a size and allocated bit into a word */10 #define PACK(size, alloc) ((size) | (alloc))1112 /* Read and write a word at address p */13 #define GET(p) (*(size_t *)(p))14 #define PUT(p, val) (*(size_t *)(p) = (val))15

Macros Figure 10.45 P746

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16 /* Read the size and allocated fields from address p */17 #define GET_SIZE(p) (GET(p) & ˜0x7)18 #define GET_ALLOC(p) (GET(p) & 0x1)1920 /* Given block ptr bp, compute address of its header and footer */21 #define HDRP(bp) ((void *)(bp) - WSIZE)22 #define FTRP(bp) ((void *)(bp) + GET_SIZE(HDRP(bp)) - DSIZE)2324 /* Given block ptr bp, compute address of next and previous blocks

*/25 #define NEXT_BLKP(bp) ((void *)(bp) + GET_SIZE(HDRP(bp)))26 #define PREV_BLKP(bp) ((void *)(bp) - GET_SIZE(((void *)(bp) -

DSIZE)))

Size_ t size = GET SIZE(HDRP(NEXT_BLKP(bp)));

Macros

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1 int mm_init(void)2 {3 /* create the initial empty heap */4 if ((heap_listp = mem_sbrk(4*WSIZE)) == NULL)5 return -1;6 PUT(heap_listp, 0); /* alignment padding */7 PUT(heap_listp+WSIZE, PACK(OVERHEAD, 1)); /* prologue header */8 PUT(heap_listp+DSIZE, PACK(OVERHEAD, 1)); /* prologue footer */9 PUT(heap_listp+WSIZE+DSIZE, PACK(0, 1)); /* epilogue header */10 heap_listp += DSIZE;1112 /* Extend the empty heap with a free block of CHUNKSIZE

bytes */13 if (extend_heap(CHUNKSIZE/WSIZE) == NULL)14 return -1;15 return 0;16 }

mm_init() Figure 10.46 P747

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1 static void *extend_heap(size_t words)2 {3 char *bp;4 size_t size;56 /* Allocate an even number of words to maintain alignment */7 size = (words % 2) ? (words+1) * WSIZE : words * WSIZE;8 if ((int)(bp = mem_sbrk(size)) < 0)9 return NULL;1011 /* Initialize free block header/footer and the epilogue

header */12 PUT(HDRP(bp), PACK(size, 0)); /* free block header */13 PUT(FTRP(bp), PACK(size, 0)); /* free block footer */14 PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 1)); /* new

epilogue header */1516 /* Coalesce if the previous block was free */17 return coalesce(bp);18 }

mm_init() Figure 10.47 P748

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1 void mm_free(void *bp)2 {3 size_t size = GET_SIZE(HDRP(bp));45 PUT(HDRP(bp), PACK(size, 0));6 PUT(FTRP(bp), PACK(size, 0));7 coalesce(bp);8 }9

mm_free() Figure 10.48 P749

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10 static void *coalesce(void *bp)11 {12 size_t prev_alloc = GET_ALLOC(FTRP(PREV_BLKP(bp)));13 size_t next_alloc = GET_ALLOC(HDRP(NEXT_BLKP(bp)));14 size_t size = GET_SIZE(HDRP(bp));1516 if (prev_alloc && next_alloc) { /* Case 1 */17 return bp;18 }1920 else if (prev_alloc && !next_alloc) { /* Case 2 */21 size += GET_SIZE(HDRP(NEXT_BLKP(bp)));22 PUT(HDRP(bp), PACK(size, 0));23 PUT(FTRP(bp), PACK(size,0));24 return(bp);25 }26

mm_free()

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27 else if (!prev_alloc && next_alloc) { /* Case 3 */28 size += GET_SIZE(HDRP(PREV_BLKP(bp)));29 PUT(FTRP(bp), PACK(size, 0));30 PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0));31 return(PREV_BLKP(bp));32 }3334 else { /* Case 4 */35 size += GET_SIZE(HDRP(PREV_BLKP(bp))) +36 GET_SIZE(FTRP(NEXT_BLKP(bp)));37 PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0));38 PUT(FTRP(NEXT_BLKP(bp)), PACK(size, 0));39 return(PREV_BLKP(bp));40 }41 }

mm_free() Figure 10.48 P749

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1 void *mm_malloc (size_t size)2 {3 size_t asize; /* adjusted block size */4 size_t extendsize; /* amount to extend heap if no fit */5 char *bp;67 /* Ignore spurious requests */8 if (size <= 0)9 return NULL;1011 /* Adjust block size to include overhead and alignment

reqs. */12 if (size <= DSIZE)13 asize = DSIZE + OVERHEAD;14 else15 asize = DSIZE * ((size + (OVERHEAD) + (DSIZE-1)) /

DSIZE);16

mm_malloc() Figure 10.49 P750

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17 /* Search the free list for a fit */18 if ((bp = find_fit(asize)) != NULL) {19 place (bp, asize);20 return bp;21 }2223 /* No fit found. Get more memory and place the

block */24 extendsize = MAX (asize, CHUNKSIZE) ;25 if ((bp = extend_heap (extendsize/WSIZE)) == NULL)26 return NULL;27 place (bp, asize);28 return bp;29 }

mm_malloc()

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1. static void *find_fit(size_t asize)2. {3. void *bp ;4. 5. /* first fit search */6. for (bp = heap_listp; GET_SIZE(HDRP(bp)) > 0 ; bp =

NEXT_BLKP(bp) ) {7. if (!GET_ALLOC(HDRP(bp)) &&

(asize<=GET_SIZE(HDRP(bp)))) {8. return bp;9. }10. }11. return NULL; /*no fit */12. }

mm_alloc() problem 10.8

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1. static void place(void *bp, size_t asize)2. {3. size_t csize = GET_SIZE(HDRP(bp)) ;4. 5. if ( (csize –asize) >= (DSIZE + OVERHEAD) ) {6. PUT(HDRP(bp), PACK(asize, 1)) ;7. PUT(FTRP(bp), PACK(asize, 1)) ;8. bp = NEXT_BLKP(bp) ;9. PUT(HDRP(bp), PACK(csize-asize, 0) ;10. PUT(FTRP(bp), PACK(csize-asize, 0) ;11. } else {12. PUT(HDRP(bp), PACK(csize, 1) ;13. PUT(FTRP(bp), PACK(csize, 1) ;14. }15. }

mm_alloc() problem 10.9

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10.9.13 Explicit Free Lists

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Explicit free lists

• Explicit list among the free blocks using pointers within the free blocks

• Use data space for link pointers– Typically doubly linked– Still need boundary tags for coalescing– It is important to realize that links are not

necessarily in the same order as the blocks

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Explicit free lists

A B C

4 4 4 4 66 44 4 4Forward links

Back links

A B

C

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Freeing with explicit free lists

• Where to put the newly freed block in the free list– LIFO (last-in-first-out) policy

• insert freed block at the beginning of the free list• pro: simple and constant time• con: studies suggest fragmentation is worse than

address ordered.

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Freeing with explicit free lists

• Where to put the newly freed block in the free list– Address-ordered policy

• insert freed blocks so that free list blocks are always in address order

– i.e. addr(pred) < addr(curr) < addr(succ)• con: requires search• pro: studies suggest fragmentation is better than

LIFO

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10.9.14 Segregated Free Lists

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Segregated Storage

• Each size “class” has its own collection of blocks– Often have separate collection for every small

size (2,3,4,…)– For larger sizes typically have a collection for

each power of 2

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Segregated Storage

1-2345-89-16

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• Separate heap and free list for each size class

• No splitting• To allocate a block of size n:

– if free list for size n is not empty,• allocate first block on list (note, list can be implicit or

explicit)– if free list is empty,

• get a new page • create new free list from all blocks in page• allocate first block on list

– constant time

1) Simple segregated storage

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• To free a block:– Add to free list– If page is empty, return the page for use by

another size (optional)

• Tradeoffs:– fast, but can fragment badly

Simple segregated storage

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• Array of free lists, each one for some size class

2) Segregated fits

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• To allocate a block of size n:– search appropriate free list for block of size m

> n– if an appropriate block is found:

• split block and place fragment on appropriate list (optional)

– if no block is found, try next larger class– repeat until block is found– if no blocks is found in all classes, try more

heap memory

Segregated fits

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• To free a block:– coalesce and place on appropriate list

(optional)• Tradeoffs

– faster search than sequential fits (i.e., log time for power of two size classes)

– controls fragmentation of simple segregated storage

– coalescing can increase search times• deferred coalescing can help

Segregated fits

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• A special case of segregated fits– Each size is power of 2

• Initialize– A heap of size 2m

3) Buddy Systems

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• Allocate– Roundup to power of 2 such as 2k

– Find a free block of size 2j (k j m)– Split the block in half until j=k

• Each remaining half block (buddy) is placed on the appreciate free list

• Free– Continue coalescing with the free buddies

Buddy Systems

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10.10 Garbage Collection

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10.10.1 Garbage Collector Basics

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• Nodes– Root Nodes– Heap Nodes

• Reachable• Not-reachable (garbage)

– Directed Edge

Garbage Collector’s View of Memory as a Directed Graph

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• Maintain some representation of the reachability graph

• Periodically reclaim the unreachable nodes by freeing them and returning them to the free list

Garbage Collector

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• Each reachable block is correctly identified as reachable

• Some unreachable nodes might be incorrectly identified as reachable

Conservative Garbage Collector

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Garbage Collector

• On demand• Run as separate threads in parallel with the

application

A Bmalloc()Conservative

garbage collector

free()AC application program

Dynamic storage allocator

Figure 10.52 P756

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10.10.2 Mark&Sweep Garbage Collectors

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Functions

• prt isPtr(ptr p)• int blockMarked(ptr b)• int blockAllocated(ptr b)• void markBlock(ptr b)• int length(b)• void unmarkBlock(ptr b)• prt nextBlock(ptr b)

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10.10.3 Conservative Mark&Sweep for C Programs

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10.11 Common Memory-Related Bugs in C Programs

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10.11.1 Dereferencing Bad Pointers

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Example

• scanf(‘’%d”, &val);• scanf(‘’%d”, val);

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10.11.2 Reading Uninitialized Memory

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Error

• Assume heap memory is initialized to zero

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10.11.3 Allowing Stack Buffer Overflows

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Buffer Overflow Bug

void bufoverflow(){

char buf[64];

gets(buf);return;

}

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10.11.4 Assuming that Pointers and the Objects they Point to Are the Same Size

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Common Mistake

• Assume that pointers to objects are the same size as the objects they point to

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10.11.5 Making Off-by-One Errors

102

Overwriting Bugs

int **makeArray2(int n, int m){

int I;int **A=(int **)Malloc(n*sizeof(int));

for (i=0; i<=n; i++)A[i] = (int *)Malloc(m*sizeof(int));

return A;}

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10.11.6 Referencing a Pointer Instead of the Object it Points to

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Precedence and associativity of C operators

int *binheapDelete(int **binheap, int *size){

int *packet = binheap[0];

binheap[0] = binheap[*size-1];*size--;heapify(binheap, *size, 0);return(packet);

}

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10.11.7 Misunderstanding Pointer Arithmetic

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Common mistake

int *search(int *p, int val){

while (*p && *p != val)p += sizeof(int);

return p;}

• Forget that arithmetic operations on pointers are performed in units that are the size of the objects they point to, which are not necessarily bytes

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10.11.8 Referencing Nonexisted Variables

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Discipline

int *stackref(){

int val;

return &val;}

• The stack discipline will sometimes reference local variables that are no longer valid

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10.11.9 Referencing Data in Free Heap Blocks

110

Error

int *heapref(int n, int m){

int i;int *x, *y;

x = (int *) Malloc(n * sizeof(int));

free(x);

y = (int *) Malloc(m * sizeof(int));for (i=0; i<m; i++)y[i] = x[i]++;return y;

}

• To reference data in heap blocks that have already been freed

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10.11.10 Introducing Memory Leaks

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Error

void leak (int n){

int *x = (int *) Malloc(n * sizeof(int));

return;}

• Create garbage in the heap by forgetting to free all allocated blocks

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10.12 Recapping Some Key Ideas About Virtual Memory

114

10.13 Summary

dynamic memory allocation

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