1. Unit 6: Real Memory organization management
Operating Systems
Operating Systems
Unit 6:
Real Memory
organization
management

2. COP 5994 - Operating Systems
Memory Hierarchy
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3. Memory Management Strategies
Allocation
Single or multiple processes
Fixed or variable
Fetch
Demand or anticipatory
Decides which piece of data to load next
Placement
Decides where in main memory to place incoming data
Replacement
Decides which data to remove from main memory to make more space
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4. COP 5994 - Operating Systems
Memory Allocation
Contiguous allocation
single block of contiguous addresses
hard/impossible to find a large enough block
low overhead
Noncontiguous allocation
address space divided into segments
each segment can be placed in different location
easier to find “holes” in which a segment will fit
more processes can run simultaneously
offsets the overhead incurred by this technique
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5. Single Process: Contiguous Memory Allocation
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6. Single Process : Overlays
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7. Single Process: Memory Protection
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8. Multi Process: Fixed-Partition Allocation
Fixed-partition multiprogramming
Each active process receives a fixed-size block of memory
Processor rapidly switches between each process
Security becomes important
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9. Multi Process: Fixed-Partition Allocation
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10. Multi Process: Fixed-Partition Allocation
with address translation
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11. Multi Process: Fixed-Partition Protection
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12. Multi Process: Fixed-Partition Drawbacks
Internal fragmentation
Process does not take up entire partition,
wasting memory
Process can be too big to fit anywhere
Variable partitions designed as replacement
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13. Multi Process: Variable-Partition Allocation
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14. Variable-Partition Characteristics
processes placed where they fit
No space wasted initially
Internal fragmentation impossible
Partitions are exactly the size they need to be
External fragmentation occurs when process ends
Leaves holes too small for new processes
Eventually no holes large enough for new processes
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15. Variable-Partition: memory holes
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16. Reduce external fragmentation
Coalescing
Combine adjacent free blocks into one large block
Compaction
Rearranges memory into
one contiguous block of free space
one contiguous block of occupied space
Significant overhead
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17. COP 5994 - Operating Systems
Memory coalescing
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18. COP 5994 - Operating Systems
Memory compaction
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19. Memory Placement Strategies
Where to put incoming processes
First-fit strategy
Process placed in first hole of sufficient size found
Simple, low execution-time overhead
Best-fit strategy
Process placed in hole that leaves least unused space
More execution-time overhead
Worst-fit strategy
Process placed in hole that leaves most unused space
remaining large hole may be used by another process
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20. Memory Placement: first fit
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21. Memory Placement: best fit
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22. Memory Placement: worst fit
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23. Multi process with Memory Swapping
remove inactive processes from memory
only running process is in main memory
others temporarily moved to secondary storage
maximizes available memory
significant overhead when switching processes
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24. Multi process with Memory Swapping
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25. Multi process with Memory Swapping
Idea: keep several processes in memory
Less available memory per process
Much faster response times
Similar to paging
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26. Evolution of memory organization
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27. COP 5994 - Operating Systems
Agenda for next week:
Midterm exam !
In 2 weeks:
Chapter 10 & 11: Virtual Memory
Read ahead !
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