- 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.
- 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.
- 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.148Up 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 It then removes the references for clauses that have been retracted before the oldest access generation from the clause list as well as the secondary clauses indexes of the predicate.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).
- Control whether or not atom and clause garbage collection are executed
in a dedicated thread. The default is
true. Values for Status are
stop. The latter stops the
gcthread 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.
- 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
trail. The table below describes the Key(Value) pairs.
Current settings can be retrieved with prolog_stack_property/2.
- 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.
- 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.
- 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.
- 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.