Dynamic Memory Allocation II.ppt

1、Dynamic Memory Allocation II,Topics Explicit doubly-linked free lists Segregated free lists Garbage collection Memory-related perils and pitfalls,dmem2.ppt,CS 105 “Tour of the Black Holes of Systems!”,Keeping Track of Free Blocks,Method 1: Implicit list using lengths - links all blocksMethod 2: Expl

2、icit list among the free blocks using pointers within the free blocksMethod 3: Segregated free lists Different free lists for different size classes Method 4: Blocks sorted by size (not discussed) Can use a balanced tree (e.g. Red-Black tree) with pointers within each free block, and the length used

3、 as a key,5,4,2,6,5,4,2,6,Explicit Free Lists,Use data space for link pointers Typically doubly linked Still need boundary tags for coalescingIt is important to realize that links are not necessarily in the same order as the blocks,4,4,4,4,6,6,4,4,4,4,Forward links,Back links,A,B,C,Allocating From E

4、xplicit Free Lists,free block,pred,succ,free block,pred,succ,Before:,After: (with splitting),Freeing With Explicit Free Lists,Insertion policy: Where in the free list do you put a newly freed block? LIFO (last-in-first-out) policy Insert freed block at the beginning of the free list Pro: simple, and

5、 constant-time Con: studies suggest fragmentation is worse than address-ordered. Address-ordered policy Insert freed blocks so free-list blocks are always in address order i.e. addr(pred) addr(curr) addr(succ)Con: requires searchPro: studies suggest fragmentation is better than LIFO,Freeing With a L

6、IFO Policy,Case 1: a-a-a Insert self at beginning of free listCase 2: a-a-f Remove next from free list, coalesce self and next, and add to beginning of free list,pred (p),succ (s),self,a,a,p,s,self,a,f,before:,p,s,f,a,after:,Freeing With a LIFO Policy (cont),Case 3: f-a-a Remove prev from free list,

7、 coalesce with self, and add to beginning of free listCase 4: f-a-f Remove prev and next from free list, coalesce with self, and add to beginning of list,p,s,self,f,a,before:,p,s,f,a,after:,p1,s1,self,f,f,before:,f,after:,p2,s2,p1,s1,p2,s2,Summary of Explicit Lists,Comparison to implicit lists: Allo

8、cate is linear-time in number of free blocks instead of total blocksmuch faster allocates when most of memory full Slightly more complicated allocate and free since needs to splice blocks in and out of free list Some extra space for links (2 extra words for each block) Main use of linked lists is in

9、 conjunction with segregated free lists Keep multiple linked lists of different size classes, or possibly for different types of objects,Keeping Track of Free Blocks,Method 1: Implicit list using lengths - links all blocksMethod 2: Explicit list among the free blocks using pointers within the free b

10、locksMethod 3: Segregated free list Different free lists for different size classes Method 4: Blocks sorted by size Can use a balanced tree (e.g. Red-Black tree) with pointers within each free block, and the length used as a key,5,4,2,6,5,4,2,6,Segregated Storage,Each size class has its own collecti

11、on of blocks,Often have separate size class for every small size (2,3,4,) For larger sizes typically have a size class for each power of 2,Simple Segregated Storage,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, alloc

12、ate 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 To free a block: Add to free list If page is empty, return the page for use by another size (optional) Tradeo

13、ffs: Fast, but can fragment badly,Segregated Fits,Array of free lists, each one for some size class 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,

14、try next larger class Repeat until block is found 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 De

15、ferred coalescing can help,Buddy Allocators,Special case of segregated fits Basic idea:Limited to power-of-two sizesCan only coalesce with “buddy“, who is other half ofnext-higher power of two Clever use of low address bits to find buddies Problem: large powers of two result in large internal fragme

16、ntation (e.g., what if you want to allocate 65537 bytes?),For More Info on Allocators,D. Knuth, “The Art of Computer Programming, Second Edition”, Addison Wesley, 1973 The classic reference on dynamic storage allocationWilson et al, “Dynamic Storage Allocation: A Survey and Critical Review”, Proc. 1

17、995 Intl Workshop on Memory Management, Kinross, Scotland, Sept, 1995. Comprehensive survey Available from CS:APP student site (csapp.cs.cmu.edu),Implicit Memory Management: Garbage Collection,Garbage collection: automatic reclamation of heap-allocated storageapplication never has to free,Common in

18、functional languages, scripting languages, and modern object oriented languages: Lisp, ML, Java, Perl, Python, Mathematica, Variants (conservative garbage collectors) exist for C and C+ Cannot collect all garbage,void foo() int *p = malloc(128);return; /* p block is now garbage */ ,Garbage Collectio

19、n,How does the memory manager know when memory can be freed? In general we cannot know what is going to be used in the future, since it depends on conditionals But we can tell that certain blocks cannot be used if there are no pointers to themNeed to make certain assumptions about pointers Memory ma

20、nager can distinguish pointers from non-pointers All pointers point to the start of a block Cannot hide pointers (e.g., by coercing them to an int and then back again),Classical GC algorithms,Mark-and-sweep collection (McCarthy, 1960) Does not move blocks (unless you also “compact”) Reference counti

21、ng (Collins, 1960) Does not move blocks (not discussed) Copying collection (Minsky, 1963) Moves blocks (not discussed) Multiprocessing compactifying (Steele, 1975)For more information, see Jones and Lin, “Garbage Collection: Algorithms for Automatic Dynamic Memory”, John Wiley & Sons, 1996.,Memory a

22、s a Graph,We view memory as a directed graph Each block is a node in the graph Each pointer is an edge in the graph Locations not in the heap that contain pointers into the heap are called root nodes (e.g. registers, locations on the stack, global variables),Root nodes,Heap nodes,Not reachable (garb

23、age),Reachable,A node (block) is reachable if there is a path from any root to that node. Non-reachable nodes are garbage (never needed by the application),Assumptions For This Lecture,Application new(n): returns pointer to new block with all locations cleared read(b,i): read location i of block b i

24、nto register write(b,i,v): write v into location i of block bEach block will have a header word Addressed as b-1, for a block b Used for different purposes in different collectorsInstructions used by the Garbage Collector is_ptr(p): determines whether p is a pointer length(b): returns length of bloc

25、k b, not including header get_roots(): returns all the roots,Mark-and-Sweep Collecting,Can build on top of malloc/free package Allocate using malloc until you “run out of space” When “out of space“: Use extra mark bit in the head of each block Mark: Start at roots and set mark bit on all reachable m

26、emory Sweep: Scan all blocks and free blocks that are not marked,Before mark,root,After mark,After sweep,free,Mark bit set,free,Mark-and-Sweep (cont.),ptr mark(ptr p) if (!is_ptr(p) return; / do nothing if not pointerif (markBitSet(p) return / quit if already markedsetMarkBit(p); / set the mark bitf

27、or (i=0; i length(p); i+) / mark all childrenmark(pi); return; ,Mark using depth-first traversal of the memory graph,Sweep using lengths to find next block,ptr sweep(ptr p, ptr end) while (p end) if markBitSet(p) clearMarkBit();else if (allocateBitSet(p) free(p);p += length(p); ,Conservative Mark-an

28、d-Sweep in C,A conservative collector for C programs Is_ptr() determines if a word is a pointer by checking if it points to an allocated block of memory. But in C, pointers can point to the middle of a block. So how do we find the beginning of the block? Can use balanced tree to keep track of all al

29、located blocks where the key is the location Balanced tree pointers can be stored in header (use two additional words),header,ptr,head,data,left,right,size,Memory-Related Bugs,Dereferencing bad pointers Reading uninitialized memory Overwriting memory Referencing nonexistent variables Freeing blocks

30、multiple times Referencing freed blocks Failing to free blocks,Dereferencing Bad Pointers,The classic scanf bug,scanf(“%d”, val);,Reading Uninitialized Memory,Assuming that heap data is initialized to zero,/* return y = Ax */ int *matvec(int *A, int *x) int *y = malloc(N*sizeof(int);int i, j;for (i=

31、0; iN; i+)for (j=0; jN; j+)yi += Aij*xj;return y; ,Overwriting Memory,Allocating the (possibly) wrong-sized object,int *p;p = malloc(N*sizeof(int);for (i=0; iN; i+) pi = malloc(M*sizeof(int); ,Overwriting Memory,Off-by-one error,int *p;p = malloc(N*sizeof(int *);for (i=0; i=N; i+) pi = malloc(M*size

32、of(int); ,Overwriting Memory,Not checking the max string sizeBasis for classic buffer-overflow attacks 1988 Internet worm Modern attacks on Web servers AOL/Microsoft IM war,char s8; int i;gets(s); /* reads “123456789” from stdin */,Overwriting Memory,Referencing a pointer instead of the object it po

33、ints to,int *BinheapDelete(int *binheap, int *size) int *packet;packet = binheap0;binheap0 = binheap*size - 1;*size-;Heapify(binheap, *size, 0);return(packet); ,Overwriting Memory,Misunderstanding pointer arithmetic,int *search(int *p, int val) while (*p ,Referencing Nonexistent Variables,Forgetting

34、 that local variables disappear when a function returns,int *foo () int val;return ,Freeing Blocks Multiple Times,Nasty!,x = malloc(N*sizeof(int);free(x);y = malloc(M*sizeof(int);free(x);,Referencing Freed Blocks,Evil!,x = malloc(N*sizeof(int);free(x); . y = malloc(M*sizeof(int); for (i=0; iM; i+)yi

35、 = xi+;,Failing to Free Blocks (Memory Leaks),Slow, long-term killer!,foo() int *x = malloc(N*sizeof(int);.return; ,Failing to Free Blocks (Memory Leaks),Freeing only part of a data structure,struct list int val;struct list *next; ;foo() struct list *head = malloc(sizeof(struct list);head-val = 0;he

36、ad-next = NULL;.free(head);return; ,Dealing With Memory Bugs,Conventional debugger (gdb) Good for finding bad pointer dereferences Hard to detect the other memory bugsDebugging malloc (CSRI UToronto malloc) Wrapper around conventional malloc Detects memory bugs at malloc and free boundaries Memory o

37、verwrites that corrupt heap structures Some instances of freeing blocks multiple times Memory leaks Cannot detect all memory bugs Overwrites into the middle of allocated blocks Freeing block twice that has been reallocated in the interim Referencing freed blocks,Dealing With Memory Bugs (cont.),Bina

38、ry translator (Atom, Purify) Powerful debugging and analysis technique Rewrites text section of executable object file Can detect same errors as debugging malloc Can also check each individual reference at runtime Bad pointers Overwriting Referencing outside of allocated block Virtual machine (Valgrind) Same power, features as binary translator Also detects references to uninitialized variables Easier to use, but slower Garbage collection (Boehm-Weiser Conservative GC) Let the system free blocks instead of the programmer.,