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Unit 6: Real Memory organization management
Operating Systems Operating Systems Unit 6: Real Memory organization management
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COP 5994 - Operating Systems
Memory Hierarchy COP Operating Systems
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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 COP Operating Systems
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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 COP Operating Systems
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Single Process: Contiguous Memory Allocation
COP Operating Systems
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Single Process : Overlays
COP Operating Systems
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Single Process: Memory Protection
COP Operating Systems
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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 COP Operating Systems
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Multi Process: Fixed-Partition Allocation
COP Operating Systems
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Multi Process: Fixed-Partition Allocation
with address translation COP Operating Systems
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Multi Process: Fixed-Partition Protection
COP Operating Systems
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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 COP Operating Systems
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Multi Process: Variable-Partition Allocation
COP Operating Systems
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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 COP Operating Systems
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Variable-Partition: memory holes
COP Operating Systems
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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 COP Operating Systems
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COP 5994 - Operating Systems
Memory coalescing COP Operating Systems
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COP 5994 - Operating Systems
Memory compaction COP Operating Systems
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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 COP Operating Systems
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Memory Placement: first fit
COP Operating Systems
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Memory Placement: best fit
COP Operating Systems
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Memory Placement: worst fit
COP Operating Systems
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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 COP Operating Systems
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Multi process with Memory Swapping
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Multi process with Memory Swapping
Idea: keep several processes in memory Less available memory per process Much faster response times Similar to paging COP Operating Systems
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Evolution of memory organization
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COP 5994 - Operating Systems
Agenda for next week: Midterm exam ! In 2 weeks: Chapter 10 & 11: Virtual Memory Read ahead ! COP Operating Systems
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