The design of a virtual fact base for Prolog
Abstract
The fact and rule list internal to Prolog is capable of
handling as many facts as available memory resources permit.
A solution to this limitation is to store facts on disk,
retrieving them into a main memory database buffer only as
needed. Allocating a fixed portion of main memory to buffer
database facts frees up scarce main memory for more
frequently accessed rules and data structures internal to
Prolog. The Prolog Database System built in connection with
this project transparently stores and retrieves facts on
disk and evaluates them in the order they were asserted
allowing for the transfer of existing small scale prototypes
into large scale production systems.
Since existing relational database techniques were not designed to function in a Prolog environment where facts are evaluated in database facilities were designed, developed, and integrated into Prolog. These database facilities include a unique page replacement policy designed to minimize expensive page faults during the execution of a Prolog program. The look ahead page replacement policy looks ahead on database pages while they are in main memory in order to determine whether they are likely to be accessed again in the future. In this way, a near optimal working set of database pages is maintained in the database buffer, assisting with minimizing expensive page faults.