PLOGHELP SYSTEM                            Jonathan Laventhol  July 1983
                                Revised by Kathryn Seifert  October 1986

Predicates which affect the way the Prolog system operates

    prolog_memlim(<prologvar>|<integer>)
    prolog_area_lim(<prologvar>|<integer>)
    prolog_area_size(<prologvar>|<integer>)
    prolog_callstack_lim(<prologvar>|<integer>)
    prolog_memused(<prologvar>)
    trimcore
    prolog_gcratio(<prologvar>|<integer>)
    prolog_gctrace(<prologvar>|on|off)
    statistics
    prolog_lock_code
    prolog_unlock_code
    prolog_no_clauses(<prologvar>|on|off)
    prolog_syspredicates(<prologvar>|on|off)
    prolog_system_predicate(<atom>, <integer>)
    version
    version(<string>)

Keywords: system predicates, system, garbage collection,
          memory allocation, compilation


CONTENTS - (Use <ENTER> g to access sections)

 -- INTRODUCTION
 -- MEMORY ALLOCATION
 -- ALTERING UPPER LIMIT ON MEMORY WITH 'PROLOG_MEMLIM'
 -- ALTERING THE LIMIT OF PROLOG TRAIL AND CONTINUATION STACKS
 -- ALTERING THE CALLSTACK LIMIT
 -- FINDING OUT HOW MUCH MEMORY WAS USED
 -- FORCING A GARBAGE COLLECTION TO OCCUR
 -- CONTROLLING GARBAGE COLLECTION
 -- BEING INFORMED ABOUT GARBAGE COLLECTIONS
 -- FINDING OUT THE SYSTEM'S STATE AT LAST GARBAGE COLLECTION
 -- STOPPING COLLECTION OF FIXED STRUCTURES
 -- SAVING SPACE AND COMPILATION TIME
 -- SAVING RUNNING TIME
 -- IDENTIFYING SYSTEM PREDICATES
 -- WHICH VERSION IS THIS?
 -- RELATED DOCUMENTATION


-- INTRODUCTION -------------------------------------------------------

There are a number of Prolog predicates which affect the way the system
operates and provide information about the system.  These predicates can
be used to control compilation, memory allocation, and garbage
collection.  There are also predicates to find out whether a predicate
is part of the system and to find out what version of Prolog is being
run.


-- MEMORY ALLOCATION --------------------------------------------------

Users are referred to the POP-11 help file HELP * POPMEMLIM for
information about memory allocation.


-- ALTERING UPPER LIMIT ON MEMORY WITH 'PROLOG_MEMLIM' ----------------

There is an upper limit on the amount of memory that the system will
expand to.  You can examine this figure, or alter it using the
predicate 'prolog_memlim'.

To discover the current limit on the memory, type:

    ?- prolog_memlim(X).
    X = 148800
    ? 
    yes

To alter the memory limit, give an integer amount to the predicate
'prolog_memlim':

    ?- prolog_memlim(400000).
    yes


-- ALTERING THE LIMIT OF PROLOG TRAIL AND CONTINUATION STACKS ---------

The Prolog trail and continuation stacks have their own limit.  The
predicate 'prolog_area_lim' can be used to examine or alter this limit.

To discover the current limit, type:

    ?- prolog_area_lim(X).
    X = 16000
    ? 
    yes

To alter this limit, give an integer amount to the predicate
'prolog_area_lim':

    ?- prolog_area_lim(25000).
    yes

The predicate prolog_area_size/1 sets the absolute size of the Prolog
memory area:

    ?- prolog_area_size(25000).
    yes

    ?- prolog_area_size(X).
    X = 25000 ?

If the size is greater than the limit set previously by prolog_area_lim/1,
the limit is increased accordingly. An error results if there is
insufficient memory to allocate the requested space.


-- ALTERING THE CALLSTACK LIMIT ---------------------------------------

There is also a limit on the space used by the procedure calling stack.
Heavily recursive procedures may need to expand this limit to avoid the
error

    ;;; PROLOG ERROR - RLE: RECURSION LIMIT (pop_callstack_lim) EXCEEDED

The limit is controlled by the predicate prolog_callstack_lim/1. You can
examine the current limit by doing

    ?- prolog_callstack_lim(X).
    X = 90000 ?
    yes

and you can change it by giving an integer as argument, as in:

    ?- prolog_callstack_lim(120000).
    yes

The limit is interpreted as a number of machine words.


-- FINDING OUT HOW MUCH MEMORY WAS USED -------------------------------

You can see how much memory was used at the last garbage collection
using the predicate 'prolog_memused':

    ?- prolog_memused(X).
    X = 4244575
    ? 
    yes


-- FORCING A GARBAGE COLLECTION TO OCCUR ------------------------------

You can cause a garbage collection by using the predicate 'trimcore'.
This predicate takes no arguments:

    ?- trimcore.
    yes


-- CONTROLLING GARBAGE COLLECTION -------------------------------------

The amount of memory in use is determined by the ratio of garbage
collection time to total CPU time. You can find out what this ratio
is, or alter it with the predicate 'prolog_gcratio'.

To find out what this ratio is, type:

    ?- prolog_gcratio(X).
    X = 25
    ? 
    yes


To alter this ratio, give an integer setting to the predicate
'prolog_gcratio':

    ?- prolog_gcratio(30).
    yes


-- BEING INFORMED ABOUT GARBAGE COLLECTIONS ---------------------------

You can have garbage collections announce themselves with the predicate
prolog_gctrace/1.

To switch the announcement statement on, type:

    ?- prolog_gctrace(on).
    yes

After this, at each subsequent garbage collection (whether it happens
automatically or is forced by using trimcore/0) you will see a message
something like:

;;; GC-auto(C) TIME: 0.27, MEM: used 19270 + free 11450 + stack 0 = 30720
;;;            CALLSTACK: 248
;;;            PROLOG: trail 2 + free 2005 + contn 41 = 2048

This indicates that the collection took 0.27 seconds, and that at the
end of it there were 19270 words of memory used and 11450 free, making a
total of 30720. The 'stack' figure shows how many words were used by the
POP-11 user stack (none in this example), and CALLSTACK shows the space
occupied on the procedure calling stack. The line beginning PROLOG gives
a breakdown of space usage in the Prolog memory area: here there were 2
words used on the trail, 41 on the continuation stack and 2005 free
making a total of 2048.

To switch tracing off again, type:

    ?- prolog_gctrace(off).
    yes

You can find out whether tracing is currently on or off by doing:

    ?- prolog_gctrace(X).
    X = off ?
    yes

Any other arguments given to 'prolog_gctrace' will fail.


-- FINDING OUT THE SYSTEM'S STATE AT LAST GARBAGE COLLECTION ----------

If you just want to see what the state of things were at the last garbage
collection, you can get a summary using the predicate 'statistics'.
This predicate takes no arguments.

    ?- statistics.
    CPU time used: 51.8 seconds. GC time: 0.15 seconds.
    At last garbage collection: 9892 words used and 390108 words free.
    yes


-- STOPPING COLLECTION OF FIXED STRUCTURES ----------------------------

You can reduce the amount of time spent in garbage collection by locking
part of the code, with the predicate "prolog_lock_code".  When this goal
is called, all of the structures in the heap will be kept, and will not
be looked at by the garbage collector.  So after you have compiled all
your predicates, and won't be changing them, you can stop the garbage
collector from wasting time looking at them.  A garbage collection is
forced, so as not to lock garbage into the heap.

To lock the code, type:

    ?- prolog_lock_code.
    ;;; GC-user TIME: 0.75, MEM: used 9484 + free 94964 + stack 0 = 104448
    ;;;            CALLSTACK: 292
    ;;;            PROLOG: trail 7 + free 2008 + contn 33 = 2048
    yes

    ?- prolog_unlock_code.
    yes


-- SAVING SPACE AND COMPILATION TIME ----------------------------------

You can reduce the space occupied by compiled predicates and save some
compilation time by using the predicate 'prolog_no_clauses/1'. This
alters things so that the predicates you define from then on will not
have their textual form recorded. You can get more information about
this by reading PLOGHELP * NO_CLAUSES.

Discarding the textual form of your definitions means you will not be
able to use the predicates 'listing' and 'clause' on them. Nor will you
be able to retract them, and if you use "assert", then it will behave
like reconsult, i.e. you will lose the previous clauses. But, your code
will compile a bit faster. If the textual form of clauses is not being
recorded, all the clauses for a given predicate must be contiguous, or
only the last contiguous sequence of clauses will be kept.

To switch the 'no_clauses' compilation mode on, type:

    ?- prolog_no_clauses(on).

and to switch it off again, type:

    ?- prolog_no_clauses(off).

You can examine the current setting by doing

    ?- prolog_no_clauses(X).
    X = off ?
    yes

The 'no_clauses' declaration described in PLOGHELP * NO_CLAUSES is a
more flexible tool for achieving this same end, as it allows the text to
be discarded from particular predicates without making a global change
to the compilation mode.


-- SAVING RUNNING TIME ------------------------------------------------

You can make your predicates run faster by having them defined as
"constant". This will mean you can't redefine them; you will also not be
able to 'spy' them (see PLOGHELP * DEBUG). In addition, the predicates
'assert' and 'retract' (see PLOGHELP * DATABASE) will not work if
predicates are defined as "constant".  You will have to define your
predicates in contiguous clauses, within the same file, otherwise you
will get an error. They will behave exactly like system predicates.

You can define particular predicates as constants by means of the
declaration 'system_predicate' described in PLOGHELP * SYSTEM_PREDICATE.
Alternatively, you can change the compilation mode globally so that all
definitions are compiled as constants by using the predicate
'prolog_syspredicates/1'. To invoke this facility, type:

    ?- prolog_syspredicates(on).
    yes

To return things to normal, type:

    ?- prolog_syspredicates(off).
    yes

You can find out the current state of things by doing:

    ?- prolog_syspredicates(X).
    X = off ?
    yes


-- IDENTIFYING SYSTEM PREDICATES --------------------------------------

You can find out if something is a system predicate using the predicate
'prolog_system_predicate'.  Calls of this predicate take the following
form:

    ?- prolog_system_predicate(<atom:P>, <integer:A>).

Where P is the predicate of a Prolog structure, and A is its arity.

Here are two examples of this predicate being used:

    ?- prolog_system_predicate(member, 2).
    no

    ?- prolog_system_predicate(prolog_system_predicate, 2).
    yes


-- WHICH VERSION IS THIS? ---------------------------------------------

The two predicates version/0 and version/1 are used to print out and
update "version" information about the Prolog system currently being
run. See PLOGHELP *VERSION for more information.


-- RELATED DOCUMENTATION ----------------------------------------------

PLOGHELP * EFFICIENCY
    Hints on how to make Prolog programs more efficient.

PLOGHELP * POPLOG
    Features of POPLOG Prolog

REF * PROLOG
    Includes information about Prolog memory organisation.


--- C.all/plog/help/system ---------------------------------------------
--- Copyright University of Sussex 1990. All rights reserved. ----------
