1 Memory Management Chapter Basic memory management 4.2 Swapping (εναλλαγή) 4.3 Virtual memory (εικονική/ιδεατή μνήμη) 4.4 Page replacement algorithms (αντικατάσταση σελιδών) 4.5 Modeling page replacement algorithms 4.6 Design issues for paging systems 4.7 Implementation issues 4.8 Segmentation (κατάτμηση)

2 Memory Management “Programs expand to fill the memory available to hold them” The part of the OS that manages memory is called memory manager. Tasks: –which parts of memory are in use and which are not –allocate/deallocate memory to processes –manage swapping between main memory and disk when memory is not large enough to hold all the processes

3 Memory Management Ideally programmers want memory that is –large –fast –non volatile Memory hierarchy –small amount of fast, expensive memory – cache –some medium-speed, medium price main memory –gigabytes of slow, cheap disk storage Memory manager handles the memory hierarchy

4 Basic Memory Management Monoprogramming without Swapping or Paging Three simple ways of organizing memory - an operating system with one user process

5 Multiprogramming with Fixed Partitions Fixed memory partitions –separate input queues for each partition (internal fragment., delays) –single input queue (alternative: favor the largest) Simple to understand, implement, run to completion (OS360)

6 Relocation and Protection Multiprogramming introduces two problems: –Rellocation: when a program is linked, the linker must know at what address the program will begin in memory. For example, call to a procedure at relative address 100. One solution: modify during loading. –Protection: One process can access the address space of another process. One solution (IBM): divide memory into blocks of 2K and assign a 4-bit protection code to each block.

7 Relocation and Protection Best solution: use base and limit hardware registers –address locations added to the base value register before sent to memory (to map to physical address) –address locations larger than base+limit value is an error Additional advantage: processes sometimes move within memory after they have started execution: all that is needed is to change the value of the base register

8 Modeling Multiprogramming One model: 20% of the time a process is in memory, it computes => 5 processes 100% CPU utilization (unrealistic). Probabilistic viewpoint: –each process spends a fraction p in I/O state –if n processes, Prob(CPU idle) = p n –CPU utilization = 1 – p n –n : degree of multiprogramming

9 Modeling Multiprogramming CPU utilization as a function of number of processes in memory Degree of multiprogramming

10 Modeling Multiprogramming This model just an approximation: processes are not independent (two processes can not run concurrently) This model can also be used for batch systems

11 Analysis of Multiprogramming System Performance Arrival and work requirements of 4 jobs CPU utilization for 1 – 4 jobs with 80% I/O wait Sequence of events as jobs arrive and finish –note numbers show amout of CPU time jobs get in each interval

12 Swapping (Εναλλαγή) So far, processes remain in main memory until they are done (loaded once). With a large number of processes we need swapping: moving processes from/to main memory to/from disk. Fixed partitions could be used, but … Therefore, we use variable partitions (size, location and number varies dynamically with number of processes)

13 Swapping Memory allocation changes as –processes come into memory –leave memory Shaded regions are unused memory

14 Swapping (Εναλλαγή) Variable partitions = flexibility, added complexity Memory compaction = moving all processes downwards (special hardware) Size of the allocated space for a process is an issue (processes can grow – malloc). If adjacent holes exist => OK, otherwise it has to be moved, or another process has to be moved, or, simply, killed. Allocate some extra memory.

15 Swapping Allocating space for growing data segment Allocating space for growing stack & data segment

16 Memory Management with Bit Maps Part of memory with 5 processes, 3 holes (a) –tick marks show allocation units –shaded regions are free Corresponding bit map (b) The size of the allocation is a design issue. When a process must be brought in, the bit map must be searched for k consecutive 0 bits

17 Memory Management with Linked Lists Part of memory with 5 processes, 3 holes Linked list of allocated and free memory segments, a segment is a process or a hole between 2 processes. This example: segment list is sorted by address; updating the list is straightforward when in/out.

18 Memory Management with Linked Lists Four neighbor combinations for the terminating process X

19 Memory Management with Linked Lists Processes and holes can be kept in a list sorted by address. Possible algorithms for de/allocation: –First fit –Next fit –Best fit –Worst fit Processes and holes can be kept in separate lists to speed up allocation – overhead in deallocation The hole list can be kept sorted on size. Quick fit: separate lists for some common sizes

20 Memory Management with Buddies The memory manager maintains a list of free blocks of size 1,2,4,8,16 bytes up to the size of the memory. (1M memory = 21 lists). Initially all of memory is free and the 1M list has a single entry containing a single 1M hole. As memory requests are coming in, lists are broken down to a power of 2 large enough to grant the request.

21 Memory Management with Buddies A A 128 A B B B B D D 256 C C C C C Initially Request 60 Request 35 Request 80 Return A Request 70 Return B Return D Return C Internal fragmentation External fragmentation

22 Analysis of Swapping Systems Allocation of swap space –Disk swap area: somewhere in swap area/specific place Analyze external fragmentation Average process after the system has come to equilibrium Half of the operations above it will be process allocations, half will be process deallocations => half of the time it has another process, half of the time has a hole. Averaged over time if n processes=> n/2 holes (50% rule) Unused memory rule: –f: fraction of memory occupied by holes –k: k>0 such that if s is the avg process size, ks is the avg hole size Then: f = k / k+2 (e.g. if k=1/2, then f = 0.2 – wasted memory)

23 Virtual Memory In swapping processes swap in/out because they block for I/O (or for other reasons – scheduling). In virtual memory, all processes are virtually in main memory. Physically, only part of them. Programs too big to fit in memory => broken down to overlays (stored in disk). OS did the swapping, user did the splitting => virtual memory. Example: 1M program can run on a 256K machine by carefully choosing the 256K overlays. Virtual memory fits well with multiprogramming (why?)

24 Virtual Memory - Paging Memory address space: MOVE REG,1000. Virtual addresses, virtual address space. No virtual memory => address requests directly in the bus Virtual memory => address requests go to MMU (memory management unit) that maps the virtual address onto a physical memory address.

25 Virtual Memory Paging The position and function of the MMU

26 Paging The relation between virtual addresses and physical memory addres- ses given by page table Example: 16-bit addresses from 0 up To 64K (virtual addresses). Physical Memory 0 to 32K (15-bit). Page size is 4K. Examples: -MOVE REG,0 -MOVE REG,8192 -MOVE REG,21500 (20 bytes in page 5) -MOVE REG,32780 (12 bytes in page 8) = page fault

27 Virtual Memory - Paging Page fault: –MMU notices that the page is unmapped and cause the CPU to trap to the Operating System. –the OS chooses a little-used page frame and puts it in the disk –the OS marks the corresponding virtual page as unmapped –replace the cross at page fault virtual page as mapped and –re-execute the trapped instruction

28 Page Tables Internal operation of MMU with 16 4 KB pages

29 Virtual Memory - Paging Two issues: –The page table can be extremely large –The mapping must be FAST Consider virtual addresses of 32 bits (or even 64!) –4-KB pages => page table has 1M pages Each instruction needs to do 1,2 or even more page table references! One solution: an array of registers like in the previous fig.

30 Page Tables 32 bit address with 2 page table fields Two-level page tables Second-level page tables Top-level page table

31 Page Tables Typical page table entry

32 TLBs – Translation Lookaside Buffers A TLB to speed up paging

33 Inverted Page Tables Comparison of a traditional page table with an inverted page table