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Memory Management

Memory Management. 4.1 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

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Memory Management

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  1. Memory Management 4.1 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) Chapter 4

  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 ManagementMonoprogramming 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) = pn • CPU utilization = 1 – pn • 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 Initially 1024 Request 70 256 A 128 512 • Internal fragmentation • External fragmentation 256 A Request 35 B 512 64 512 64 C A B 128 Request 80 128 C B 64 128 Return A 512 Request 60 D 128 B C 512 128 64 D 512 Return B 128 C 128 256 Return D 128 512 C Return C 1024

  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 MemoryPaging The position and function of the MMU

  26. Paging The relation betweenvirtual addressesand physical memory addres-ses given bypage 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 Second-level page tables • 32 bit address with 2 page table fields • Two-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

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