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# Memory Management Glossary: E

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ecru

  See

  off-white.

edge

  In a graph, an edge is a connection between two nodes.

  In a directed graph (digraph), edges have a direction; otherwise the start and end nodes are interchangeable. By convention, two directed edges between the same two nodes, but in different directions, are depicted as a bi-directional edge.

  Typically an edge represents some relation between nodes.

  Relevance to memory management

  In memory management, edges normally represent the fact that an object holds a reference to another object.

  See also

  graph.

entry table(1)

  An entry table is a table of references into a set of objects used to indirect references from the outside.

  The Lieberman-Hewitt collector(1) represented references from older generations to younger ones by indirect pointers through an entry table in the younger generation that contained the actual address of the young object. This is fairly expensive without special hardware; other generational collectors generally use remembered sets.

  See also

  generational garbage collection, exit table.

  Related publication

  Lieberman & Hewitt (1983).

entry table(2)

  An entry table is an implementation of a remembered set, where, for a given generation, there is a list of objects in older generations which contain references into that generation.

  One could also store the actual locations of the references, which would save time when scanning, but incur other costs.

  Similar term

  remembered set.

  See also

  generational garbage collection, exit table.

exact garbage collection

  Also known as

  precise garbage collection, type-accurate garbage collection.

  Garbage collection is exact (or precise) if it deals only with exact references.

  An exact collector(1) needs to know the format of the objects and the roots, so that it can tell which fields are references.

  Opposite term

  conservative garbage collection.

exact reference

  Also known as

  precise reference, sure reference.

  An exact or precise or sure reference is a value the collector(1) knows is a reference.

  This is the usual sort of reference. The term is used to draw a contrast with ambiguous reference.

  Opposite term

  ambiguous reference.

exact root

  Also known as

  precise root.

  An exact or precise root is a root that contains only exact references.

  Opposite term

  ambiguous root.

  See also

  exact reference.

  In the MPS

  An exact root has rank `mps_rank_exact()`.

exact segregated fit

  A segregated fit allocation mechanism which has a separate free list for each possible block size. The array of free lists may be represented sparsely. Large blocks may be treated separately.

  See also

  segregated fit, segregated free list, allocation mechanism.

  Related publication

  Wilson et al. (1995).

execution stack

  See

  control stack.

exit table

  An exit table is a table of all references from a set of objects to objects outside the set.

  See also

  entry table(1), entry table(2).

  Related publication

  Lieberman & Hewitt (1983).

extent

  See

  lifetime.

external fragmentation

  External fragmentation is the inability to use memory(1) because free(3) memory is divided into many small blocks.

  If live objects are scattered, the free blocks cannot be coalesced, and hence no large blocks can be allocated.

  Common solutions to external fragmentation include:

  1. Moving garbage collection;
  2. Handles;
  3. Making all your objects the same size.

  See also

  internal fragmentation.

  Related publication

  Johnstone & Wilson (1998).

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