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Tables can both be private to a thread or shared between all threads. Private tables are used only by the calling threads and are discarded as the thread terminates. Shared tables are used by all threads and can only be discarded explicitly. Tables are declared as shared using, e.g.,
:- table (p/1, q/2) as shared.
A thread may find a a table for a particular variant of a shared tabled predicate in any of the following states:
deadlockexception. This exception causes the current SCC to be abandonned and gives other threads the opportunity to claim ownership of the tables that were owned by this thread. The thread that raised the exception and abandonned the SCC simply restarts the leader goal of the SCC. As other threads now have claimed more variants of the SCC it will, in most cases, wait for these threads instead of creating a new deadlock.
A thread that waits for a table may be faced with three results. If
the table is complete it can use the answers. It is also possible that
the thread that was filling the table raised an exception (either a
deadlock or any other exception), in which case we find a
fresh table for which we will try to claim ownership. Finally,
some thread may have abolished the table. This situation is the same as
when the owning thread raised an exception.
This section briefly explains the interaction between deleting shared tables and running threads. The core rule is that abolishing a shared table has no effect on the semantics of the tabled predicates. An attempt to abolish an incomplete table results in the table to be marked for destruction on completion. The thread that is completing the table continues to do so and continues execution with the computed table answers. Any other thread blocks, waiting for the the table to complete. Once completed, the table is destroyed and the waiting threads see a fresh table165Future versions may avoid waiting by converting the abolished shared table to a private table..
The current implementation never reclaims shared tables. Instead, they remain part of the global variant table and only the answers of the shared table are reclaimed. Future versions may garbage collect such tables.
Currently, shared tabling has many restrictions. The implementation does not verify that the limitations are met and violating these restrictions may cause incorrect results or crashes. Future versions are expected to resolve these issues.
SWI-Prolog's continuation based tabling offers the opportunity to perform completion using multiple threads.