Dynamic Memory Allocation: Advanced Concepts

References

Slides adapted from CMU

Review: Dynamic Memory Allocation

Programmers use dynamic memory allocators (such as malloc) to acquire virtual memory (VM) at run time.
- For data structures where the size is only known at runtime
Dynamic memory allocators manage an area of process VM known as the heap.

Review: Keeping Track of Free Blocks

Method 1: Implicit list using length; links all blocks
- Need to tag each block as allocated/free
Method 2: Explicit list among the free blocks using pointers
- Need space for pointers
Method 3: Segregated free list
- Different free lists for different size classes
Method 4: Blocks sorted by size
- Can use a balanced tree with pointers within each free block, and the length used as a key

Review: Implicit Lists Summary

Implementation: very simple
Allocate cost: linear time worst case
Free cost: constant time worst case (even with coalescing)
Memory overhead: depends on placement policy
Not used in practice for malloc/free because of linear time allocation
The concepts of splitting and boundary tag coalescing are general to all allocators

Explicit Free Lists

Maintain list(s) of free blocks, not all blocks
- We track only free blocks, so we can use payload area
- The “next” free block could be anywhere
  - We need to store forward/backward pointers, not just sizes
- Still need boundary tags for coalescing
  - To find adjacent blocks according to memory order

Freeing With Explicit Free Lists

Insertion policy: where in the free list do you put a newly freed block?
Unordered
- LIFO (last-in-first-out) policy
  - Insert freed block at the beginning of the free list
- FIFO (first-in-first-out) policy
  - Insert freed block at the end of the free list
- Pro: simple and constant time
- Con: studies suggest fragmentation is worse than address ordered
Address-ordered policy
- Insert freed blocks so that free list blocks are always in address order:
  \(addr(prev) < addr(curr) < addr(next)\)
- Con: requires search
- Pro: studies suggest fragmentation is lower than LIFO/FIFO

Freeing with a LIFO Policy

Case 1: allocated \(\leftrightarrow\) target \(\leftrightarrow\) allocated
- Insert the freed block at the root of the list
Case 2: allocated \(\leftrightarrow\) target \(\leftrightarrow\) free
- Splice out adjacent successor block, coalesce both memory blocks, and insert the new block at the root of the list
Case 3: free \(\leftrightarrow\) target \(\leftrightarrow\) allocated
- Splice out adjacent successor block, coalesce both memory blocks, and insert the new block at the root of the list
Case 4: free \(\leftrightarrow\) target \(\leftrightarrow\) free
- Splice out adjacent successor block, coalesce all three memory blocks, and insert the new block at the root of the list

An Implementation Trick

Use circular, doubly linked list
Support multiple approaches with single data structure
First-fit versus next-fit
- Either keep free pointer fixed or move as search list
LIFO versus FIFO
- Insert as next block (LIFO) or previous block (FIFO)

Explicit List Summary

Comparison to implicit list:
- Allocate is linear time in number of free blocks instead of all blocks (much faster)
- Slightly more complicated allocate and free because we need to splice blocks in and out of the list
- Some extra space for the links (two extra words needed for each block)
  - Does this increase internal fragmentation?

Segregated List (Seglist) Allocators

Each size class of blocks has its own free list
- Example size classes: 16, 32-48, 64-inf
Often have separate classes for each small size
For larger sized: one class for each size \([2^{i} + 1, 2^{i+1}]\)

Seglist Allocator

Given an 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 \(m > n\) (that is, first fit)
- If an appropriate block is found:
  - Split block and place fragment on appropriate list
  - If no block is found, try next larger class
- Repeat until block is found
If no block is found:
- Request additional heap memory from OS (using sbrk())
- Allocate block of \(n\) byte from this new memory
- Place remainder as single free block in appropriate size class

Seglist Allocator (Continued)

To free a block:
- Coalesce and place on appropriate list
Advantages of seglist allocators versus non-seglist allocators (both with first fit)
- Higher throughput
  - log time for power-of-two size classes versus linear time
- Better memory utilization
  - First fit search of segregated free list approximates a best fit search of the entire heap
  - Extreme case: giving each block its own size class is equivalent to best fit

Dereferencing bad pointers
Reading uninitialized memory
Overwriting memory
Referencing nonexistent variables
Freeing blocks multiple times
Referencing freed blocks
Failing to free blocks

Dereferencing Bad Pointers

The classic scanf bug
```
int val;

...

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)); // <-- here
   int i, j;

   for (i=0; i<N; i++)
      for (j=0; j<N; j++)
         y[i] += A[i][j]*x[j];
   return y;
}

Can avoid by using calloc

Overwriting Memory

Allocating the (possibly) wrong sized object

int **p;

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

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

Can you spot the bug?

Overwriting Memory

Off-by-one errors

char **p;

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

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

char *p;

p = malloc(strlen(s));
strcpy(p,s);

Overwriting Memory

Not checking the max string size

char s[8]; // <-- too small
int i;

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

Basis for classic buffer overflow attacks

Overwriting Memory

Misunderstanding pointer arithmetic

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

   while (p && *p != val)
      p += sizeof(int); // <-- here

   return p;
}

Overwriting Memory

Referencing a pointer instead of the object it points to

int *BinheapDelete(int **binheap, int *size) {
   int *packet;
   packet = binheap[0];
   binheap[0] = binheap[*size - 1];
   *size--;  // <-- here
   Heapify(binheap, *size, 0);
   return(packet);
}

What gets decremented?
- Hint: precedence and associativity

Referencing Nonexistent Variables

Forgetting that local variables disappear when a function returns
```
int *foo () {
   int val;

   return &val;
}
```

Freeing Blocks Multiple Times

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

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

Referencing Freed Blocks

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

Failing to Free Blocks (Memory Leaks)

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;
   head->next = NULL;
   <create and manipulate the rest of the list>
    ...
   free(head);
   return;
}

Dealing With Memory Bugs

Debugger: gdb
- Good for finding bad pointer dereferences
- Hard to detect the other memory bugs
Data structure consistency checker
- Runs silently, prints message only on error
- Use as a probe to zero in on error
Binary translator: valgrind
- Powerful debugging and analysis technique
- Rewrites text section of executable object file
- Checks each individual reference at runtime
  - Bad pointers, overwrites, refs outside of allocated block
glibc malloc contains checking code

Implicit Memory Management: Garbage Collection

Garbage collection: automatic reclamation of heap-allocated storage; application never has to explicitly free memory
Common in many dynamic languages
Variants (“conservative” garbage collectors) exist for C and C++

Garbage Collection

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 them
Must make certain assumptions about pointers
- Memory manager can distinguish pointers from non-pointers
- All pointers point to the start of a block
- Cannot hide pointers (for example, coercing them to an int and then back again)

Classical Garbage Collection Algorithms

Mark-and-sweep collection (McCarthy, 1960)
- Does not move blocks (unless you also “compact”)
Reference counting (Collins, 1960)
- Does not move blocks (not discussed)
Copying collection (Minsky, 1963)
- Moves blocks (not discussed)
Generational Collectors (Lieberman and Hewitt, 1983)
- Collection based on lifetimes
  - Most allocations become garbage very soon
  - So, focus reclamation work on zones of memory recently allocated

Memory as 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 (for example, registers, locations on the stack, global variables)

Memory as a Graph

A node (block) is reachable if there is a path from any root to that node
Non-reachable nodes are garbage (cannot be needed by the application)

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 each reachable block
- Sweep: scan all blocks and free blocks that are not marked

Assumptions for a Simple Implementation

Application
- 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 a header word
- addressed as b[-1], for block b
- used for different purposes in different collectors
Instructions used by the Garbage Collector
- is_ptr(p): determines whether p is a pointer
- length(b): returns the length of block b, not including the header
- get_roots(): returns all the roots

Mark and Sweep Pseudocode

Mark using depth-first traversal of the memory graph

ptr mark(ptr p) {
   if (!is_ptr(p)) return;        // if not pointer -> do nothing
   if (markBitSet(p)) return;     // if already marked -> do nothing
   setMarkBit(p);                 // set the mark bit
   for (i=0; i < length(p); i++)  // for each word in p’s block
       mark(p[i]);                //  make recursive call
   return;
}

Mark and Sweep Pseudocode

Sweep using lengths to find next block

ptr sweep(ptr p, ptr end) {
   while (p < end) {              // for entire heap
      if markBitSet(p)            // did we reach this block?
         clearMarkBit();          //  yes -> so just clear mark bit
      else if (allocateBitSet(p)) // never reached: is it allocated?
         free(p);                 //  yes -> its garbage, free it
      p += length(p+1);           // goto next block
}

Conservative Mark and Sweep in C

A “conservative garbage 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
To mark header, need to find the beginning of the block
- Can use a balanced binary tree to keep track of all allocated blocks (key is start-of-block)
- Balanced tree pointers can be stored in header (use two additional words)

Dynamic Memory Allocation: Advanced Concepts

References

Review: Dynamic Memory Allocation

Review: Keeping Track of Free Blocks

Review: Implicit Lists Summary

Explicit Free Lists

Freeing With Explicit Free Lists

Freeing with a LIFO Policy

An Implementation Trick

Explicit List Summary

Segregated List (Seglist) Allocators

Seglist Allocator

Seglist Allocator (Continued)

Memory Related Perils and Pitfalls

Dereferencing Bad Pointers

Reading Uninitialized Memory

Overwriting Memory

Overwriting Memory

Overwriting Memory

Overwriting Memory

Overwriting Memory

Referencing Nonexistent Variables

Freeing Blocks Multiple Times

Referencing Freed Blocks

Failing to Free Blocks (Memory Leaks)

Failing to Free Blocks (Memory Leaks)

Dealing With Memory Bugs

Implicit Memory Management: Garbage Collection

Garbage Collection

Classical Garbage Collection Algorithms

Memory as a Graph

Memory as a Graph

Mark and Sweep Collecting

Assumptions for a Simple Implementation

Mark and Sweep Pseudocode

Mark and Sweep Pseudocode

Conservative Mark and Sweep in C