Dynamic Memory Allocation II

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Dynamic Memory Allocation II Topics Topics Explicit doubly-linked free lists Segregated free lists Garbage collection Memory-related perils and pitfalls CS 105 our of the Black Holes of Computi

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Dynamic Memory Allocation II. CS 105 Tour of the Black Holes of Computing. Topics Explicit doubly-linked free lists Segregated free lists Garbage collection Memory-related perils and pitfalls. Keeping Track of Free Blocks. Method 1 : Implicit list using lengths -- links all blocks - PowerPoint PPT Presentation

Transcript of Dynamic Memory Allocation II

Page 1: Dynamic Memory Allocation II

Dynamic Memory Allocation IIDynamic Memory Allocation II

TopicsTopics Explicit doubly-linked free lists Segregated free lists Garbage collection Memory-related perils and pitfalls

CS 105Tour of the Black Holes of Computing

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Keeping Track of Free BlocksKeeping Track of Free Blocks

Method 1Method 1: : Implicit listImplicit list using lengths -- links all blocks using lengths -- links all blocks

Method 2Method 2: : Explicit listExplicit list among the free blocks using among the free blocks using pointers within the free blockspointers within the free blocks

Method 3Method 3: : Segregated free listSegregated free list Different free lists for different size classes

Method 4Method 4: : Blocks sorted by size (not discussed)Blocks sorted by size (not discussed) For example balanced tree (Red-Black?) with pointers inside

each free block, block length used as key

20 16 824

20 16 824

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Explicit Free ListsExplicit Free Lists

Use data space for link pointersUse data space for link pointers Typically doubly linked Still need boundary tags for coalescing

Links aren’t necessarily in same order as blocks!

A B C

16 16 16 16 2424 1616 16 16

Forward links

Back links

A B

C

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Allocating From Explicit Free ListsAllocating From Explicit Free Lists

free block

pred succ

free block

pred succ

Before:

After:(with splitting)

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Freeing With Explicit Free ListsFreeing With Explicit Free Lists

Insertion policyInsertion policy: Where in free list to put newly freed : Where in free list to put newly freed block?block? LIFO (last-in-first-out) policy

Insert freed block at beginning of free listPro: simple, and constant-timeCon: studies suggest fragmentation is worse than address-

ordered

Address-ordered policy Insert freed blocks so list is always in address order

» i.e. addr(pred) < addr(curr) < addr(succ) Con: requires search (using boundary tags) Pro: studies suggest fragmentation is better than LIFO

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Freeing With a LIFO PolicyFreeing With a LIFO Policy

Case 1: a-a-aCase 1: a-a-a Insert self at beginning of

free list

Case 2: a-a-fCase 2: a-a-f Remove next from free

list, coalesce self and next, and add to beginning of free list

pred (p) succ (s)

selfa a

p s

selfa f

before:

p s

faafter:

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Freeing With a LIFO Policy (cont)Freeing With a LIFO Policy (cont)

Case 3: f-a-aCase 3: f-a-a Remove prev from free list,

coalesce with self, and add to beginning of free list

Case 4: f-a-fCase 4: f-a-f Remove prev and next

from free list, coalesce with self, and add to beginning of list

p s

selff a

before:

p s

f aafter:

p1 s1

selff f

before:

fafter:

p2 s2

p1 s1 p2 s2

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Summary of Explicit ListsSummary of Explicit Lists

Comparison to implicit lists:Comparison to implicit lists: Allocate is linear-time in number of free blocks instead of total

blocks—much faster 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 per block)—but can

reuse data space so no “real” cost

Main use of linked lists is in conjunction with segregated Main use of linked lists is in conjunction with segregated free listsfree lists Keep multiple linked lists of different size classes, or possibly

for different types of objects

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Keeping Track of Free BlocksKeeping Track of Free Blocks

Method 1Method 1: : Implicit listImplicit list using lengths -- links all blocks using lengths -- links all blocks

Method 2Method 2: : Explicit listExplicit list among the free blocks using among the free blocks using pointers within the free blockspointers within the free blocks

Method 3Method 3: : Segregated free listSegregated free list Different free lists for different size classes

Method 4Method 4: : Blocks sorted by size (not discussed)Blocks sorted by size (not discussed) For example balanced tree (Red-Black?) with pointers inside

each free block, block length used as key

20 16 824

20 16 824

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Segregated StorageSegregated Storage

Each Each size classsize class has its own collection of blocks has its own collection of blocks

4-8

12

16

20-32

36-64

Often separate size class for every small size (8, 12, 16, …) For larger, typically have size class for each power of 2

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Simple Segregated StorageSimple Segregated StorageSeparate Separate heap and free list for each size classheap and free list for each size class

No splittingNo splitting

To allocate block of size n:To allocate block of size n: If free list for size n is not empty,

Allocate first block on list (can be implicit or explicit)

If free list is empty, Get new page Create new free list from all blocks in page Allocate first block on list

Constant time

To free block:To free block: Add to free list If page empty, return it for use by another size (optional)

Tradeoffs:Tradeoffs: Fast, but can fragment badly

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Segregated FitsSegregated FitsArray of free lists, one for each size classArray of free lists, one for each size class

To allocate block of size n:To allocate block of size n: Search appropriate list for block of size m > n If block found, split and put fragment on smaller list (optional) If no block found, try next larger class and repeat If largest class empty, allocate page(s) big enough to hold

desired block, put remainder on appropriate list

To free a block:To free a block: Coalesce and put on appropriate list

TradeoffsTradeoffs Faster search than sequential fits (log time for power-of-two

size classes) Controls fragmentation of simple segregated storage Coalescing can increase search times

Deferred coalescing can help

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Buddy AllocatorsBuddy Allocators

Special case of segregated fitsSpecial case of segregated fits

Basic idea:Basic idea:

Limited to power-of-two sizesLimited to power-of-two sizes

Can only coalesce with "buddy", who is other half ofCan only coalesce with "buddy", who is other half of

next-higher power of twonext-higher power of two

Clever use of low address bits to find buddiesClever use of low address bits to find buddies

Problem: large powers of two result in large internal Problem: large powers of two result in large internal fragmentation (e.g., what if you want to allocate fragmentation (e.g., what if you want to allocate 65537 bytes?)65537 bytes?)

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For More Info on AllocatorsFor More Info on Allocators

D. Knuth, D. Knuth, ““The Art of Computer Programming, Second The Art of Computer Programming, Second EditionEdition””, Addison Wesley, 1973, Addison Wesley, 1973 Classic reference on dynamic storage allocation

Wilson et al, Wilson et al, ““Dynamic Storage Allocation: A Survey Dynamic Storage Allocation: A Survey and Critical Reviewand Critical Review””, Proc. 1995 Int, Proc. 1995 Int’’l Workshop on l Workshop on Memory Management, Kinross, Scotland, Sept, 1995.Memory Management, Kinross, Scotland, Sept, 1995. Comprehensive survey Available from CS:APP student site (csapp.cs.cmu.edu)

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Implicit Memory Management:Garbage CollectionImplicit Memory Management:Garbage Collection

Garbage collectionGarbage collection: : automatic reclamation of heap-automatic reclamation of heap-allocated storage—application never has to freeallocated storage—application never has to free

Common in functional languages, scripting languages, Common in functional languages, scripting languages, and modern object-oriented languages:and modern object-oriented languages: Lisp, ML, Java, Perl, Python, Mathematica, …

Variants (conservative garbage collectors) exist for C Variants (conservative garbage collectors) exist for C and C++and C++ Cannot collect all garbage

void foo() { int *p = malloc(128); return; /* p block is now garbage */}

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Garbage CollectionGarbage CollectionHow does memory manager know when memory can How does memory manager know when memory can

be freed?be freed? In general can’t know what will be used in future, since

depends on conditionals But we know certain blocks can’t be used if there are no

pointers to them

Need to make certain assumptions about pointersNeed to make certain assumptions about pointers Memory manager can distinguish pointers from non-

pointers All pointers point to start of block Can’t hide pointers (e.g., by coercing them to an int and

then back again, still a memory address)

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Classical GC algorithmsClassical GC algorithms

Mark-and-sweep collection (McCarthy, 1960)Mark-and-sweep collection (McCarthy, 1960) Doesn’t move blocks (unless you also “compact”)

Reference counting (Collins, 1960)Reference counting (Collins, 1960) Doesn’t move blocks (not discussed)

Copying collection (Minsky, 1963)Copying collection (Minsky, 1963) Moves blocks (not discussed)

Multiprocessing compactifying (Steele, 1975)Multiprocessing compactifying (Steele, 1975)

For more information, see For more information, see Jones and Lin, Jones and Lin, ““Garbage Garbage Collection: Algorithms for Automatic Dynamic Collection: Algorithms for Automatic Dynamic MemoryMemory””, John Wiley & Sons, 1996., John Wiley & Sons, 1996.

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Memory as a GraphMemory as a GraphThink of memory as directed graphThink of memory as directed graph

Each block is node in graph Each pointer is edge Locations not in heap that contain pointers into heap are called root nodes (e.g.

registers, locations on stack, global variables)

Root nodes

Heap nodes

Not reachable(garbage)

Reachable

Node (block) is Node (block) is reachable if there is path from any root to that node. if there is path from any root to that node.

Non-reachable nodes are Non-reachable nodes are garbage garbage (never needed, nor accessible by (never needed, nor accessible by application)application)

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Assumptions For This LectureAssumptions For This LectureApplicationApplication

new(n): returns pointer to new block with all locations cleared read(b,i): read location i of block b into register write(b,i,v): write v into location i of block b

Each block will have header wordEach block will have header word Addressed as b[-1], for a block b Used for different purposes in different collectors

Instructions used by garbage collectorInstructions used by garbage collector is_ptr(p): determines whether p is pointer length(b): returns length of block b, not including header get_roots(): returns all roots

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Mark-and-Sweep CollectingMark-and-Sweep CollectingCan build on top of malloc/free packageCan build on top of malloc/free package

Allocate using malloc until you “run out of space”

When "out of space":When "out of space": Use extra mark bit in head of each block Mark: Start at roots and set mark bit on all reachable memory Sweep: Scan all blocks and free the blocks that are not marked Why were the two free blocks Not on FreeList?

Before mark

root

After mark

After sweep free

Mark bit set

free

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Mark-and-Sweep (cont.)Mark-and-Sweep (cont.)

ptr mark(ptr p) { if (!is_ptr(p)) return; /* ignore non-pointers */ if (markBitSet(p)) return; /* quit if already marked */ setMarkBit(p); /* set the mark bit */ for (i=0; i < length(p); i++) /* mark all children */ mark(p[i]); return;}

Mark using depth-first traversal of 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); --fine next block}

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Conservative Mark-and-Sweep in CConservative Mark-and-Sweep in C

A conservative collector for C programsA conservative collector for C programs is_ptr() determines if word is a pointer by checking if it

points to allocated block of memory. But in C, pointers can point to middle of a block.

So how do we find beginning of block?So how do we find beginning of block? Can use balanced tree to keep track of all allocated blocks,

where key is the location Tree pointers can be stored in header (use two additional

words)

header

ptr

head data

left right

size

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Memory-Related BugsMemory-Related Bugs

Dereferencing bad pointersDereferencing bad pointers

Reading uninitialized memoryReading uninitialized memory

Overwriting memoryOverwriting memory

Referencing nonexistent variablesReferencing nonexistent variables

Freeing blocks multiple timesFreeing blocks multiple times

Referencing freed blocksReferencing freed blocks

Failing to free blocksFailing to free blocks

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Dereferencing Bad PointersDereferencing Bad Pointers

The classic The classic scanfscanf bug bug

scanf(“%d”, val);

must be a ptrto an int

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Reading Uninitialized MemoryReading Uninitialized Memory

Assuming that heap data is initialized to zeroAssuming 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=0; i<N; i++) for (j=0; j<N; j++) y[i] += A[i][j]*x[j]; return y;}

A is pointer topointer of int

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Overwriting MemoryOverwriting Memory

Allocating the (possibly) wrong-sized objectAllocating the (possibly) wrong-sized object

int **p;

p = malloc(N*sizeof(int));

for (i = 0; i < N; i++) { p[i] = malloc(M*sizeof(int));}

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Overwriting MemoryOverwriting Memory

Off-by-one errorOff-by-one error

int **p;

p = malloc(N*sizeof(int *));

for (i = 0; i <= N; i++) { p[i] = malloc(M*sizeof(int));}

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Overwriting MemoryOverwriting Memory

Not checking the max string sizeNot checking the max string size

Basis for classic buffer-overflow attacksBasis for classic buffer-overflow attacks 1988 Internet worm Modern attacks on Web servers AOL/Microsoft IM war

char s[8];int i;

gets(s); /* reads “123456789” from stdin */

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Overwriting MemoryOverwriting Memory

Misunderstanding pointer arithmeticMisunderstanding pointer arithmetic

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

return p;}

p is a pointerp += 1 would

add 4 to p

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Referencing Nonexistent VariablesReferencing Nonexistent VariablesForgetting that local variables disappear when a Forgetting that local variables disappear when a

function returnsfunction returns

int *foo () { int val; return &val;}

val goes awaywith end of foo

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Freeing Blocks Multiple TimesFreeing Blocks Multiple Times

Nasty!Nasty!

x = malloc(N*sizeof(int));<manipulate x>free(x);

y = malloc(M*sizeof(int));<manipulate y>free(x);

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Referencing Freed BlocksReferencing Freed Blocks

Evil! Evil!

x = malloc(N*sizeof(int));<manipulate x>free(x);...y = malloc(M*sizeof(int));for (i=0; i<M; i++) y[i] = x[i]++;

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Failing to Free Blocks(Memory Leaks)Failing to Free Blocks(Memory Leaks)Slow, long-term killer! Slow, long-term killer!

foo() { int *x = malloc(N*sizeof(int)); ... return;}

no auto cleanupin C

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Failing to Free Blocks(Memory Leaks)Failing to Free Blocks(Memory Leaks)Freeing only part of a data structureFreeing only part of a data structure

struct list { int val; struct list *next;};

foo() { struct list *head = malloc(sizeof(struct list)); head->val = 0; head->next = NULL; <create and manipulate the rest of the list> ... free(head); return;}

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Dealing With Memory BugsDealing With Memory Bugs

Conventional debugger (Conventional debugger (gdbgdb)) Good for finding bad pointer dereferences Hard to detect the other memory bugs

Debugging Debugging mallocmalloc (CSRI UToronto (CSRI UToronto mallocmalloc)) Wrapper around conventional malloc Detects memory bugs at malloc and free boundaries

Memory overwrites that corrupt heap structuresSome instances of freeing blocks multiple timesMemory leaks

Cannot detect all memory bugsOverwrites into the middle of allocated blocksFreeing block twice that has been reallocated in the interimReferencing freed blocks

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Dealing With Memory Bugs (cont.)Dealing With Memory Bugs (cont.)Binary translator (Atom, Purify)Binary 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)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)Garbage collection (Boehm-Weiser Conservative GC) Let the system free blocks instead of the programmer.

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The EndThe End

DefinitionsDefinitions Malloc Garbage Collection Stride

ProblemsProblems 9.6 9.7 9.10 9.19