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Garbage Collection

Garbage Collection. records not reachable reclaim to allow reuse performed by runtime system (support programs linked with the compiled code). Record Types. live – will be used in the future not live – will not be used in the future reachable – able to be accessed via programs.

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Garbage Collection

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  1. Garbage Collection • records not reachable • reclaim to allow reuse • performed by runtime system (support programs linked with the compiled code)

  2. Record Types • live – will be used in the future • not live – will not be used in the future • reachable – able to be accessed via programs

  3. Types of Algorithms • Mark-And-Sweep Collection • Reference Counts • Copying Collection • Generational Collection • Incremental Collection • Baker’s Algorithm

  4. Mark-And-Sweep Collection • Program variables and heap records form a directed graph • Roots are the variables • node n is reachable if r -> … -> n • Depth first search marks reachable nodes • Any node not marked is garbage

  5. Cost of Garbage Collection • Depth first search takes time proportional to the number of reachable nodes • Sweep phase takes time proportional to the size of the heap

  6. Maintaining Free Space • Create a list of free space • Search for a space of size N might be long • Maintain several free lists of differing sizes • External fragmentation a problem • Internal fragmentation can also be a problem

  7. Reference Counts • Count the number of pointers point to each record • Store the reference count with each record • If p addresses an alternate record, decrement the old and increment the new • If count reaches 0, free record

  8. When to Decrement Instead of decrementing the counts a record references when the record is placed on the free list, it is better to do this when the record is removed from the free list.

  9. Why • Breaks the recursive decrementing work into shorter pieces • Compiler emits code to check whether the count has reached 0, but the recursive decrementing will be done only in one place, in the allocator

  10. Problems with Reference Count • Cycles of garbage cannot be reclaimed • Incrementing the reference counts is very expensive

  11. Solutions-Cycles, Expensive • Require the programmer to break the cycle • Combine reference counting with mark-sweep • No solution for it being expensive • Problems outweigh advantages, thus rarely used

  12. Copying Collection • Reachable part is a directed graph with records as nodes, pointers as edges, and variables as roots • Copy the graph from “from-space” to “to-space” • Delete all “from-space”qq

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