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GC16/3011 Functional Programming Lecture 18 Automatic Memory Management

GC16/3011 Functional Programming Lecture 18 Automatic Memory Management. Contents. What is AMM? Programmer control vs system control Reuse/recycling of memory How does AMM work? Memory allocation Garbage collection Issues How is garbage created, detected, reused?

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GC16/3011 Functional Programming Lecture 18 Automatic Memory Management

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  1. GC16/3011 Functional ProgrammingLecture 18Automatic Memory Management

  2. Contents • What is AMM? • Programmer control vs system control • Reuse/recycling of memory • How does AMM work? • Memory allocation • Garbage collection • Issues • How is garbage created, detected, reused? • What overheads do we incur? (space/time) • Fragmentation

  3. What is AMM? • Biggest source of bugs: POINTERS • Big cost: • Effective management (reuse) of memory • A solution: • Don’t let programmers have direct access to memory locations (NO POINTERS) • Let system manage memory • Functional languages, Java • An onerous responsibility for the system • must never go wrong

  4. How does AMM work? • A storage manager (SM) subroutine services requests from the rest of the program • Program requests “N bytes of memory” • SM: • searches for appropriate chunk of “free” memory • Allocates the chunk (tags it “in use” or “live”) • returns a pointer to that chunk • Programmer never sees the pointer • SM detects when “in use” chunk becomes garbage and tags it “free”

  5. How does AMM work? (2) • Memory allocation techniques • Which chunk (block) of memory should the SM return in response to a request? Does it matter? • Garbage collection techniques • How to identify garbage • How to collect garbage • Compaction/defragmentation techniques • How does fragmentation occur • How can it be reduced or removed

  6. Issues: Garbage collection • How is garbage created? • Beta reduction, delta reduction … • How is garbage identified? • Number of references? … or connectivity • How is garbage collected? • Use a free list? … or not • How is garbage reused? • Cooperation with memory allocation

  7. Issues: how much does it cost? • Time • Performance degradation • Embarrassing pause? • Real-time systems? • Space • Some memory set aside for administration? • Some extra memory required per cell? • Size of code?

  8. Issues: fragmentation • What is it? • Why is it a problem? • Embedded systems • Virtual memory – paging overhead • How can it be solved? • Coalescing • Compaction: • Copying • Sliding

  9. Summary • What is AMM? • Programmer control vs system control • Reuse/recycling of memory • How does AMM work? • Memory allocation • Garbage collection • Issues • How is garbage created, detected, reused? • What overheads do we incur? (space/time) • Fragmentation

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