1. Dynamic Memory Allocation
CSCE 410/611
Dynamic Memory Allocation
Memory Allocation: where?
Fragmentation
external vs. internal
Implementation Approaches
Naïve vs. Buddy System
Dynamic Memory Management

2. Dynamic Memory Allocation – when?
CSCE 410/611
Dynamic Memory Allocation – when?
When does new memory get allocated in the kernel?
New process gets created (fork())
New program gets loaded into memory (execve())
Process stack grows
What about thread stacks?
Process expands heap (malloc())
Process “creates” (attaches to) shared memory segment (shmat())
Map a file to memory (mmap())
Note: These memory segments “belong to” the user!
For internal memory management OS uses Slab Allocator.
Dynamic Memory Management

3. libraries & dev.drivers
CSCE 410/611
Memory Allocation
memory space
user program area
libraries & dev.drivers
Monitor
Job1
Job2
Job3
Job4
Dynamic Memory Management

4. Memory Allocation X First Fit Worst Fit Best Fit CSCE 410/611
Dynamic Memory Management

5. (External) Fragmentation
CSCE 410/611
(External) Fragmentation
allocate() ?!
Dynamic Memory Management

6. (Internal) Fragmentation
CSCE 410/611
(Internal) Fragmentation
Internal Fragmentation
Dynamic Memory Management

7. (Internal) Fragmentation
CSCE 410/611
(Internal) Fragmentation
Dynamic Memory Management

8. Memory Allocation: Data Structures
CSCE 410/611
Memory Allocation: Data Structures
global
memory descriptor
next
memory regions
start
end
memory space
start
end
next
free list
Dynamic Memory Management

9. (Binary) Buddy System Allocation
CSCE 410/611
(Binary) Buddy System Allocation
Maintain multiple free lists, one for each power-of-2 size.
Allocation:
Increase size of request to next power of 2*.
Look up segment in free lists.
If exists, allocate.
If none exists, split next larger segment in half, put first half (the “buddy”) on free list, and return second half.
Harry Markowitz
1927-
1990 Nobel Memorial Prize in Economics
(nobelprize.org)
De-Allocation:
Return segment to free list.
Check if buddy is free. If so, coalesce.
References: Donald Knuth: The Art of Computer Programming Volume 1: Fundamental Algorithms. Second Edition (Reading, Massachusetts: Addison-Wesley, 1997), pp ISBN
Dynamic Memory Management

10. How to find a buddy?
The buddy of each block can be found with an Exclusive OR of the block’s address and the block’s size.
Ex) 32KB size =
First 32KB Block = XOR

11. Allocation at Different Levels
user level
explicit user-level allocation:
malloc(size)
allocate virtually contiguous sequence of bytes at user level
user-level library
kernel level
byte-sized allocations:
kmalloc(size, gfp_mask)
allocate physically contiguous sequence of bytes
vmalloc(size, gfp_mask)
allocate virtually contiguous sequence of bytes
Slab Allocator
(+ caching)
alloc_pages() and __get_free_pages()
allocate frames or pages, at low level
useful to allocate contiguous frames/pages.
Buddy System!

12. Summary: Dynamic Memory Allocation
CSCE 410/611
Summary: Dynamic Memory Allocation
Memory Allocation
Internal and External Fragmentation
Implementation Approaches
Free Lists
Buddy System
Dynamic Memory Management