4.42.1 Garbage collection

garbage_collect

Invoke the global and trail stack garbage collector. Normally the garbage collector is invoked automatically if necessary. Explicit invocation might be useful to reduce the need for garbage collections in time-critical segments of the code. After the garbage collection trim_stacks/0 is invoked to release the collected memory resources.

garbage_collect_atoms

Reclaim unused atoms. Normally invoked after agc_margin (a Prolog flag) atoms have been created. On multithreaded versions the actual collection is delayed until there are no threads performing normal garbage collection. In this case garbage_collect_atoms/0 returns immediately. Note that there is no guarantee it will ever happen, as there may always be threads performing garbage collection.

garbage_collect_clauses

Reclaim retracted clauses. During normal operation, retracting a clause implies setting the erased generation to the current generation of the database and increment the generation. Keeping the clause around is both needed to realise the logical update view and deal with the fact that other threads may be executing the clause. Both static and dynamic code is processed this way.^{150Up to version 7.3.11, dynamic code was handled using reference counts.}.

The clause garbage collector (CGC) scans the environment stacks of all threads for referenced dirty predicates and at which generation this reference accesses the predicate. It then removes the references for clauses that have been retracted before the oldest access generation from the clause list as well as the secondary clauses indexes of the predicate. If the clause list is not being scanned, the clause references and ultimately the clause itself is reclaimed.

The clause garbage collector is called under three conditions, (1) after reloading a source file, (2) if the memory occupied by retracted but not yet reclaimed clauses exceeds 12.5% of the program store, or (3) if skipping dead clauses in the clause lists becomes too costly. The cost of clause garbage collection is proportional with the total size of the local stack of all threads (the scanning phase) and the number of clauses in all‘dirty' predicates (the reclaiming phase).

set_prolog_gc_thread(+Status)

Control whether or not atom and clause garbage collection are executed in a dedicated thread. The default is true. Values for Status are true, false and stop. The latter stops the gc thread but allows is to be recreated lazily. This is use by e.g., fork/1 to avoid forking a multi-threaded application. See also gc_thread.

trim_stacks

Release stack memory resources that are not in use at this moment, returning them to the operating system. It can be used to release memory resources in a backtracking loop, where the iterations require typically seconds of execution time and very different, potentially large, amounts of stack space. Such a loop can be written as follows:

loop :-
        generator,
            trim_stacks,
            potentially_expensive_operation,
        stop_condition, !.

The Prolog top-level loop is written this way, reclaiming memory resources after every user query.

set_prolog_stack(+Stack, +KeyValue)

Set a parameter for one of the Prolog runtime stacks. Stack is one of local, global or trail. The table below describes the Key(Value) pairs.

Current settings can be retrieved with prolog_stack_property/2.

min_free(+Cells)

Minimum amount of free space after trimming or shifting the stack. Setting this value higher can reduce the number of garbage collections and stack-shifts at the cost of higher memory usage. The amount is reported and specified in cells. A cell is 4 bytes in the 32-bit version and 8 bytes on the 64-bit version. See address_bits. See also trim_stacks/0 and debug/0.

low(+Cells)

factor(+Number)

These two figures determine whether, if the stacks are low, a stack shift (expansion) or garbage collection is performed. This depends on these two parameters, the current stack usage and the amount of stack used after the last garbage collection. A garbage collection is started if used > factor × lastused + low.

spare(+Cells)

All stacks trigger overflow before actually reaching the limit, so the resulting error can be handled gracefully. The spare stack is used for print_message/2 from the garbage collector and for handling exceptions. The default suffices, unless the user redefines related hooks. Do not specify large values for this because it reduces the amount of memory available for your real task.

Related hooks are message_hook/3 (redefining GC messages), prolog_trace_interception/4 and prolog_exception_hook/4.

prolog_stack_property(?Stack, ?KeyValue)

True if KeyValue is a current property of Stack. See set_prolog_stack/2 for defined properties.

The total space limit for all stacks is controlled using the prolog flag stack_limit.