Indexing policy

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Indexing policies determine the locations to which a block is mapped based on its address.

The most important methods of indexing policies are getPossibleEntries() and regenerateAddr():

  • getPossibleEntries() determines the list of entries a given address can be mapped to.
  • regenerateAddr() uses the address information stored in an entry to determine its full original address.

For further information on Cache Indexing Policies, please refer to the wikipedia articles on Placement Policies and Associativity.

Set Associative

The set associative indexing policy is the standard for table-like structures, and can be further divided into Direct-Mapped (or 1-way set-associative), Set-Associative and Full-Associative (N-way set-associative, where N is the number of table entries).

A set associative cache can be seen as a skewed associative cache whose skewing function maps to the same value for every way.

Skewed Associative

The skewed associative indexing policy has a variable mapping based on a hash function, so a value x can be mapped to different sets, based on the way being used. Gem5 implements skewed caches as described in "Skewed-Associative Caches", from Seznec et al.

Note that there are only a limited number of implemented hashing functions, so if the number of ways is higher than that number then a sub-optimal automatically generated hash function is used.