Memory Management. Frédéric Haziza Spring Department of Computer Systems Uppsala University

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1 Memory Management Frédéric Haziza Department of Computer Systems Uppsala University Spring 2008

2 Operating Systems Process Management Memory Management Storage Management Compilers Compiling process & Lexical analysis Parsing Semantic & Code generation

3 Recall Memory Management CPU MEM Disk Which processes and data to move in and out memory Allocating and deallocating memory space as needed Keeping track of which parts of memory are currently being used and by whom

4 Outline 1 Considerations Memory is central Speed Protection 2 Address Binding 3 Memory Allocation Fixed-sized partition Variable-sized partition Algorithms Fragmentation 4 OSKomp 08 Memory Management

5 Typical: 1 Fetch instruction form MEM 2 Decode instruction 3 Fetch eventual operands from MEM 4 Execute instruction 5 Store eventual results to MEM But for the MEM: stream of addresses. No difference between data and instructions Hardware: CPU - MEM - IO Registers+MEM CPU No instruction with disk address All must be first brought to MEM 5 OSKomp 08 Memory Management

6 Speed considerations Register: 1 cycle (1 ns) Mem (via Bus & transactions): cycles (100 ns) CPU must stall (if no instruction scheduling, cf DARK2) Cache 6 OSKomp 08 Memory Management

7 Speed, ok, but Need of Protection Speed, ok, but need for protection (User process shouldn t interfer with each others) Idea Each process has a separate memory space, i.e. a range of valid addresses 7 OSKomp 08 Memory Management

8 Example: 2 registers - base & limit base base + limit CPU address yes yes < no no MEM Trap to OS monitor addressing error HW checks every address. If problem, trap. Prevents user from touching others user space or OS space. Base and limit placed in PCB. Registers loaded by OS only (since privileged instruction) OS = superpower 8 OSKomp 08 Memory Management

9 How do we bring a program in MEM? Programs are typically on disk as executable files and must be brought to MEM. Input Queue Processes on disk waiting to be brought into MEM Cf. analogy with the ready queue. 9 OSKomp 08 Memory Management

10 Translation steps source program compiler or assembler Compile time Each step: more specific translation other object modules object module linkage editor If translation at compile-time: absolute code system library dynamically loaded system library load module loader Load time If translation as run-time: done by MMU in-memory binary memory image Run time 10 OSKomp 08 Memory Management

11 Example: Relocation register (base register) Relocation register CPU logical address physical address MEM MMU Used to relocate addresses after OS space (protection for that latter) Address a becomes a + x but OS can vary in size (due to buffers,... ) x in register (dynamic relocation) added to all addresses. 11 OSKomp 08 Memory Management

12 Saving Space Dynamic loading Do we need to load everything? No, just when needed. If routine is needed, check if loaded. If not, loads the routine, stored on disk as relocatable load format Advantage: If never used, never loaded (example: error routines) No special hardware support, up to programmers to use it smartly Dynamic linking If static: Embrace system libs in all programs Programs are bigger Stub (Description page 281) Extension to library updates (bug fixes) 12 OSKomp 08 Memory Management

13 Saving More Space - Swapping 13 OSKomp 08 Memory Management

14 Saving More Space - Swapping 14 OSKomp 08 Memory Management

15 Saving More Space - Swapping example Example User process: 10 MB Backing store: standard disk with transfer rate of 40 MB/s Latency: 8ms Swap time = 258 ms Swap in and out Total swap time 516 ms If RR, quantum s 15 OSKomp 08 Memory Management

16 Saving More Space - Swapping Swap is slow Swap the necessary i.e. what the process IS using and not MIGHT BE using. Other constraint: Process must be idle If swapped out, MEM reclaimed to another process, so should not be waiting for IO Solutions: Never swap a process with pending IO Execute IO into OS buffers only (Transfer buffers to process mem, when swapped in) 16 OSKomp 08 Memory Management

17 Saving Even More Space What about the OS? Can we reclaim some of its space? Recall the OS space is protected, for example, with the relocation register. But can be used to dynamically allow OS size to change. What to toss? OS can grow & shrink: OS buffers, code for device drivers,... transcient code: comes & goes as needed 17 OSKomp 08 Memory Management

18 Fixed-sized partition 1 partition - 1 process Split memory in fixed size partitions, give one to each process. When processes terminate, memory is freed and can be reused by other processes. Problem: Degree of multiprogramming is limited to the number of partitions 18 OSKomp 08 Memory Management

19 Variable-sized partition Recall that processes waiting to be swapped in form the input queue (on disk). OS checks mem requirements for processes Job Scheduling: Input queue is sorted with an algorithm (cf CPU scheduling) Take the first one or skip down the list to find one where mem requirements can be met Risk? Starvation 19 OSKomp 08 Memory Management

20 Variable-sized partition OS process 5 process 8 OS process 5 OS process 5 process 9 OS process 5 process 9 process 10 process 2 process 2 process 2 process 2 Note: Memory is contiguous 20 OSKomp 08 Memory Management

21 Algorithms First-fit Allocate the first hole that is big enough Searching stops as soon as hole found Best-fit Allocate the smallest hole that is big enough smallest leftover hole Worst-fit Allocate the largest hole Searching over the entire list (unless list is ordered) First-fit & Best-fit Worst-fit. First-fit is faster than Best-fit. But First-fit & Best-fit lead to external fragmentation 21 OSKomp 08 Memory Management

22 Fragmentation Internal space within an allocated block of memory is unused (the block is larger than necessary) External unallocated blocks of memory are unused because they are each too small, although the sum of their sizes would be usable. 22 OSKomp 08 Memory Management

23 Dealing with Fragmentation Avoid external fragmentation: Compaction(only if dynamic relocation) Cost? Permit non-contiguous address space Paging Note: What about fragmentation of backing store when swapped out? Too slow to think about compaction!! 24 OSKomp 08 Memory Management

12: Memory Management

12: Memory Management Mark Handley Address Binding Program goes through multiple steps from compilation to execution. At some stage, addresses in the program must be bound to physical memory addresses: