Operating Systems { week 12a}

1. Rensselaer Polytechnic Institute CSCI-4210 – Operating Systems David Goldschmidt, Ph.D. Operating Systems{week 12a}

2. Hierarchical storage architecture very fast very small volatile non-volatile very slow very large

3. Von Neumann architecture • Based on the von Neumann architecture, data and program instructionsexist in physical memory • Repeatedly performfetch-decode-executecycles • The execute partoften results in datafetch and store operations physical memory

4. Main memory (i) • Locations in memoryare identified bymemory addresses • When compiled, programsconsist of relocatable code • Other compiled modulesalso consist ofrelocatable code symbolic addresses in source code relative addresses in object code

5. Main memory (ii) • At load time, anyadditional librariesalso consist ofrelocatable code physical addresses generated by loader

6. Main memory (iii) • At run time, memoryaddresses of all objectfiles are mapped to asingle memory spacein physical memory

7. Dynamic loading and linking • Using dynamic loading, external libraries are not loaded when a process starts • Libraries are stored on disk in relocatable form • Libraries loaded into memory only when needed • Using dynamic linking, external libraries can be preloaded into shared memory • When a process calls a library function, thecorresponding physical address is determined

8. Contiguous memory allocation (i) • Main memoryis partitionedand allocatedto residentoperating systemand user processes fixed partitioning scheme

9. Contiguous memory allocation (ii) • A pair of base and limitregisters define thelogical address space • Also known asrelocation registers

10. Contiguous memory allocation (iii) • The CPU generates logical memory addresses • A Memory-Management Unit (MMU)maps logical memory addressesto the physical address space • User programs never seephysical memory addresses

11. Contiguous memory allocation (iv) • Hardware protects against memory access outside of a process’s valid memory space

12. OS OS OS OS Process 5 Process 5 Process 5 Process 5 Process 9 Process 9 Process 8 Process 1 Process 2 Process 2 Process 2 Process 2 Dynamic partitioning • Variable-length or dynamic partitions: • When a new process enters the system, the process is allocated to a single contiguous block • The operating system maintains a list of allocated partitions and free partitions

13. Placement algorithms • How can we place new process Pi in memory? • First-fit algorithm: allocate the first free blockthat’s large enough to accommodate Pi • Best-fit algorithm: allocate thesmallest free block that’s largeenough to accommodate Pi • Next-fit algorithm: allocate thenext free block, searching from last allocated block • Worst-fit algorithm: allocate the largest free blockthat’s large enough to accommodate Pi

14. OS Process 5 Process 8 Process 3 Process 2 Process 6 Process 7 Process 12 Fragmentation (i) • Memory is wasted due to fragmentation,which can cause performance issues • Internal fragmentation is wasted memorywithina partition or process memory • External fragmentation can reducethe number of runnable processes • Total memory space exists to satisfya memory request, but memory isnot contiguous Process 9

15. OS Process 5 Process 8 Process 3 Process 3 Process 2 Process 3 Process 6 Process 6 Process 6 Process 7 Process 7 Process 7 Process 12 Process 12 Process 12 Fragmentation (ii) • Reduce external fragmentation bycompaction or defragmentation • Rearrange memory contents to organizeall free memory blocks together intoone large contiguous block • Compaction is possible only ifrelocation is dynamic and isdone at execution time • Compaction is expensive Process 9 Process 9