Memory Allocation and Garbage Collection

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Memory Allocation andGarbage Collection
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Why Dynamic Memory?

• We cannot know memory requirements in advance
  when the program is written.
• In early Fortran all memory had to be allocated
  in advance. There was no dynamic memory
• You can request memory dynamically using
  malloc/new.

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Types of Memory Management

• Dynamic memory needs to be recycled after it is
  no longer in use or the system may run out of
  memory.
• There are two ways memory can be recycled
• Explicit Memory Management
• The program calls free/delete explicitly
• Automatic Memory Management
• The system determines what can be recycled using
  Automatic Garbage Collection

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Explicit Memory Management

• The program calls free/delete when object is no
  longer in use.
• Advantages
• It uses less memory. The program do not need to
  wait to recycle memory.
• Faster. No GC overhead and the memory allocated
  is likely to be in cache.

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Explicit Memory Management

• Disadvantages
• Error prone
• Memory leaks - Memory is never freed. It causes
  system slow down or running out of memory swap
  space. Bad 24/7 apps.
• Premature Frees Memory is freed while still in
  use. It causes the program to crash.
• Double frees Free an object that is already
  freed.
• Free of Non-Heap objects Free an object that
  was not allocated with malloc/free.
• Memory leaks is less severe than the other three
  but is still bad.

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Implicit Memory Management

• Let the system determine what memory is no longer
  in use and recycle it.
• There are two basic approaches
• Reference Counting
• Tracing

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Reference Counting

• In each object keep a counter that represents the
  number of references pointing to the object.
• When the reference counter reaches 0, it means
  that there is no reference to this object so it
  can be safely recycled.

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Reference Counting

• Advantages
• Memory is recycled as soon as it is no longer in
  use.
• Disadvantages
• cycles will not be removed because the counter
  never reaches 0.
• Each pointer assignment will need an
  increment/decrement operation in the counters.
• It is used by Perl, Phyton and some times in Java
  with auxiliar GC to collect cycles.

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Tracing Garbage Collection

• The objects in a program are divided into two
  groups
• Root objects Objects that where not allocated
  with malloc/new and that may contain reference to
  dynamic objects.
• Dynamic objects Objects tahat are allocated
  with malloc/new and can become unused during the
  execution of the program

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Tracing Garbage Collection

• Each object has a mark bit
• A stack or queue is used to keep track of the
  objects that have been not been visited.
• Tracing Garbage Collection Algorithm
• Before a GC the mark bits are cleared
• Scan all the root objects for references to
  dynamic objects. If the object referenced has not
  been marked, mark it and push reference to the
  stack.
• While the stack is not empty pop one reference
  from the stack, scan it for references and if the
  object referenced has not been marked, mark it
  and push reference to the stack.

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Tracing Garbage Collection

• Stop until the stack is empty
• When the stack is empty, the unmarked objects are
  no longer reachable from the roots and can be
  safely recycled.

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Tracing Garbage Collection
Root Objects
Marked Live
Dynamic Objects
Unmarked - Garbage