Administrivia

• “Lab 3: FAT32 Filesystem” is due on Mar 2!
• Proposal day:
  • Two days to accommodate the larger number of teams/students
  • Slides DUE on Feb 24! (2-day earlier than the original plan)

Q&A

Is there any other file systems as easy as FAT series?

Since Box uses allocate in its implementation and is considered “safe” by Rust, would it be possible to introduce/create an infinite number of Boxes which will take up all of a system’s memory and cause a memory error?

Agenda

• malloc()/free()
• Page frames (physical memory)
• Two heap/page allocators

Simple High-level Interfaces

  // allocate a memory region (an object)
  void *malloc(size_t size);
  // free a memory region
  void free(void *ptr);

  // allocate a memory region for an array
  void *calloc(size_t nmemb, size_t size);
  // resize/reallocate a memory region
  void *realloc(void *ptr, size_t size);

  // new Type == malloc(sizeof(Type))
  // new Type[size] == malloc(sizeof(Type)*size)

CS101: Heap Allocators

Q0. ptr = malloc(size); *ptr?
Q1. ptr = malloc(0); ptr == NULL?
Q2. ptr = malloc(-1); ptr == NULL?
Q3. ptr = malloc(size); ptr == NULL but valid? /* vaddr = 0 */

Q4. free(ptr); ptr == NULL?
Q5. free(ptr); *ptr?
Q6. free(NULL)?

CS101: Common Goals of Heap Allocators

1. Performance → worse case/common path of alloc/free
2. Memory fragmentation → internal / external fragmentation
3. Security (out-of-scope, but check cs6265!)

ptmalloc in Linux: Memory Allocation

ptr = malloc(size);

ptmalloc in Linux: Data Structure

struct malloc_chunk {
  [...]
  // double links for free chunks in small/large bins
  //  (only valid when this chunk is freed)
  struct malloc_chunk* fd;
  struct malloc_chunk* bk;
  
  // double links for next larger/smaller size in largebins
  //  (only valid when this chunk is freed)
  struct malloc_chunk* fd_nextsize;
  struct malloc_chunk* bk_nextsize;
};

ptmalloc in Linux: Memory Free

Common Design Choices

1. Binning: size-base groups/operations
   • e.g., caching the same size objects together
2. In-place metadata: metadata before/after or even inside
   • e.g., putting metadata inside the freed region
3. Cardinal metadata: no encoding, direct pointers and sizes
   • e.g., using raw pointers for linked lists

Understanding Modern Heap Allocators

• Maximize memory usage: using free memory regions!
• Data structure to minimize fragmentation (i.e., fd/bk consolidation)
• Data structure to maximize performance (i.e., O(1) in free/malloc)

Wait, we are the OS kernel?

• Yes, we meant to provide allocation APIs for physical pages/frames

Wait, we are the OS kernel?

• Yes, we meant to provide allocation APIs for physical pages/frames

GlobalAllocator in Rust

• Q. How is it different from malloc()/free()?
• Q. How would you do if you need one physical frame (i.e., one page)?

// in std::alloc::GlobalAlloc
unsafe impl GlobalAlloc {
  unsafe fn alloc(&self, layout: Layout) -> *mut u8 {}
  unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {}
}

pub struct Layout {
    size_: usize,
    align_: NonZeroUsize,
}

Comparing with native heap allocator

unsafe fn alloc(&self, layout: Layout) -> *mut u8 {}
unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {}

vs.,

Q0. ptr = malloc(size); *ptr?
Q1. ptr = malloc(0); ptr == NULL?
Q2. ptr = malloc(-1); ptr == NULL?
Q3. ptr = malloc(size); ptr == NULL but valid? /* vaddr = 0 */

Q4. free(ptr); ptr == NULL?
Q5. free(ptr); *ptr?
Q6. free(NULL)?

Two naive implementations

• Bump allocator: simplest form, no reuse/free
• Bin allocator: attempting to minimize fragmentation

Ref. Allocator Designs

Bump allocator

• Q. How to do alloc()?
• Q. How to do free()?
• Q. How to handle layout (i.e., alignment)?

Maintaining free chunks for reuse

• Q. How to do free()?
• Q. How to do alloc()?
• Q. Where to store “link” then?

Bin allocator

• Introducing size-based caching on freed chunks
• Maintaining a linked list per a group of free chunks with similar sizes (i.e., bin)
• Q. How to do alloc()?
• Q. How to do free()?

      Bins
sz=16 [ -]--->[fd]--->[fd]-->NULL
sz=24 [ -]--->[fd]--->NULL
sz=32 [ -]--->NULL
       ...

Problems of bin allocator

• Q. Is the external fragmentation issue resolved?
• Q. How would you respect proper layout in an efficient manner?
• Q. What if there is no proper size/layout in my bin? searching larger-size bin?

References

• Page implementation
• Heap allocation
• Allocator Designs