Memory Management

1. Memory Management Memory allocation Garbage collection

2. Memory Allocation • Memory pool: large block of contiguous memory • Memory manager allocates memory by returning a handle to the user • Use the term heap to refer to free memory accessed by a dynamic memory management scheme

3. Dynamic Allocation • Blocks of any size may be requested in any order from the free-list • For a request of m words, and a block size k, between m and k space is used for the request • This can result in fragmentation if m != k

4. Fragmentation in Dynamic Allocation • External fragmentation: lots of small free blocks • Internal fragmentation: when all of block of size k is allocated for m words. This type of allocation is easier, if less efficient

5. Sequential Fit Method • Attempt to find a “good” block • The free-list is organized as a doubly-linked list • Tag bit and block size fields • The memory manager searches the free-list for a block of “suitable” size

6. Three sequential fit methods • First fit • Start from the beginning (or middle) • May waste larger blocks by breaking them up • Best fit • Examines entire list • Maximizes external fragmentation but will be more likely to be able to service large requests

7. Three sequential fit methods • Worst fit • Allocates largest block through a sequential search • Minimizes external fragmentation • Which is best? Depends on the expected types of memory requests

8. Sequential Fit • A search of the free-list is in Ө(n) in the worst case • Want to merge adjacent free blocks • Need additional space to support the memory manager operations/linked list • Is there anything that can be improved?

9. The Buddy Method • Assume that memory is of size 2ⁿ for some n • Both free and reserved blocks will be of size 2kfor k ≤ n • The buddy system keeps a separate list of free blocks for each size

10. The Buddy Method • For a request of size m, find the smallest k where 2k ≥ m • If such a k exists, allocate a block of size 2k from the list • If such a k does not exist, allocate the next larger block on the list, and split it in half until a block of size 2k is created

11. The Buddy Method • Advantages • Less external fragmentation • Cheaper search than a linked list • Merging adjacent blocks is easy (the buddy for any block of size 2k is another block of the same size with the same address except the kth bit is reversed) • Disadvantage • Allows internal fragmentation

12. Other memory allocation methods • Segregated storage method: break available memory into several memory zones, each with its own management method • Impose a standard size (cluster scheme) • Example: disk file management • Leads to internal fragmentation • Does not need to be contiguous

13. Failure Policies • Happens when a memory request for a certain size cannot be serviced • Due to external fragmentation → compact memory, which physically moves data • Use a handle if the application relies on absolute positions of the data • Can defer the memory request (for example, when several processes are running at once)

14. Failure policies: Garbage Collection • When no program variable points to a block of space, it is considered garbage (or a memory leak) • Garbage collection involves determining which memory is garbage and recovering it • Two common methods: • Reference count • Mark/sweep strategy

15. Garbage Collection • Reference Count: each dynamically allocated memory block has a count field that is incremented and decremented for each pointer pointing to it (or away from it) • When the count reaches zero, the memory becomes garbage and is immediately placed in free store • Used by the UNIX file system, where the memory objects are linked together without cycles • Useful when the objects are large, such as a file

16. Garbage Collection • Mark/sweep strategy • Uses a single bit marker instead of a count field • Works for cycles, but DFS is recursive • Garbage collection phase occurs when free store is exhausted: • Clear all mark bits • Perform DFS from each pointer on the variable list, turning on the bits • Sweep through the memory pool for unmarked elements (which are garbage and placed in free store)