1. Lecture 26: Memory Management - Swapping

2. Review: goals of memory management
Memory allocation
A new process comes into memory
Memory de-allocation
A process leaves memory
Address translation
Virtual address to physical address
Process isolation
One process cannot access the memory of other processes

3. Review: Memory management for different systems
Processes are known in advance and can fit in memory
Embedded systems, e.g., radios, washing machines, and microwaves
Processes are unknown in advance, memory might not be large enough
Swapping
Virtual memory

4. Swapping Each individual process can fit in memory
But memory cannot fit all processes
Each process occupies some consecutive blocks
Swap some process out to disk
Swap in a process when needed

5. Managing free memory
Bitmap
Linked list

6. Bitmap Divide memory into blocks
Use a bit 0/1 for every block to indicate whether it is free or not

7. Bitmap (a) (b) A part of memory with five processes and three holes.
Shaded areas are holes
(b) The corresponding bitmap

8. Memory allocation/de-allocation with Bitmap
Needed when a new process is loaded or swapped in from disk
Suppose a process needs k blocks
Find k consecutive 0 bits in the bitmap
Memory deallocation
Reset the corresponding blocks in bitmap to 0

9. Linked list
Put processes and free memory blocks in a linked list

10. Linked list A part of memory with five processes and three holes.
Shaded areas are holes
(b) The corresponding bitmap
(c) Linked list

11. Linked list Put processes and free memory blocks in a linked list
Each node in this list has four fields
First field (binary): indicate free or not
Second field (integer): indicate the start address
Third field (integer): length of the memory segment
Fourth field (pointer): point to the next segment

12. Memory allocation in linked list
First fit
Go through the linked list, and find the first free segment that can fit the process, and then split the segment.
Best fit
Find the smallest free segment that is enough
Worst fit
Find the largest free segment

13. Memory de-allocation in linked list
Merge neighboring free segments into a larger one

14. Address translation
Static address translation slows down loading/swapping in
Dynamic address translation
Base register in CPU
Store the starting physical address of a process
Physical address = base + virtual address

15. Process isolation “Limit register” in CPU
Store the length of a process
By coming the base register and limit register, we know the memory area that a process has access to

16. Base and limit registers

17. Memory compaction
A relatively large process might not fit into the memory even if the free memory is large enough
When the free memory are divided into small non-consecutive segments
Memory compaction
Periodically move processes in memory so that we can have large free segments