2.20 System limits

2.20.1 Limits on memory areas

SWI-Prolog has a number of memory areas which are only enlarged to a certain limit. The internal data representation limits the local, global and trail stack to 128 MB on 32-bit processors, or more generally to 2 ** bits-per-pointer - 5 bytes. Considering that almost all modern hardware can deal with this amount of memory with ease, the default limits are set to their maximum on 32-bit hardware. The representation limits can easily exceed physical memory on 64-bit hardware. The default limits on 64-bit hardware are double that of 32-bit hardware, which allows for storing the same amount of (Prolog) data.

The limits can be changed from the command line as well as at runtime using set_prolog_stack/2. The table below shows these areas. The first column gives the option name to modify the size of the area. The option character is immediately followed by a number and optionally by a k or m. With k or no unit indicator, the value is interpreted in Kbytes (1024 bytes); with m, the value is interpreted in Mbytes (1024 × 1024 bytes).

The PrologScript facility described in section 2.10.2.1 provides a mechanism for specifying options with the load file. On Windows the default stack sizes are controlled using the Windows registry on the key HKEY_CURRENT_USER\Software\SWI\Prolog using the names localSize, globalSize and trailSize. The value is a DWORD expressing the default stack size in Kbytes. A GUI for modifying these values is provided using the XPCE package. To use this, start the XPCE manual tools using manpce0, after which you find Preferences in the File menu.

Considering portability, applications that need to modify the default limits are advised to do so using set_prolog_stack/2.

Option	Default	Area name	Description
-L	128M	local stack	The local stack is used to store the execution environments of procedure invocations. The space for an environment is reclaimed when it fails, exits without leaving choice points, the alternatives are cut off with the !/0 predicate or no choice points have been created since the invocation and the last subclause is started (last call optimisation).
-G	128M	global stack	The global stack is used to store terms created during Prolog's execution. Terms on this stack will be reclaimed by backtracking to a point before the term was created or by garbage collection (provided the term is no longer referenced).
-T	128M	trail stack	The trail stack is used to store assignments during execution. Entries on this stack remain alive until backtracking before the point of creation or the garbage collector determines they are no longer needed.

2.20.1.1 The heap

With the heap, we refer to the memory area used by malloc() and friends. SWI-Prolog uses the area to store atoms, functors, predicates and their clauses, records and other dynamic data. No limits are imposed on the addresses returned by malloc() and friends.

2.20.2 Other Limits

Clauses

The only limit on clauses is their arity (the number of arguments to the head), which is limited to 1024. Raising this limit is easy and relatively cheap; removing it is harder.

Atoms and Strings

SWI-Prolog has no limits on the length of atoms and strings. The number of atoms is limited to 16777216 (16M) on 32-bit machines. On 64-bit machines this is virtually unlimited. See also section 11.4.2.1.

Memory areas

On 32-bit hardware, SWI-Prolog data is packed in a 32-bit word, which contains both type and value information. The size of the various memory areas is limited to 128 MB for each of the areas, except for the program heap, which is not limited. On 64-bit hardware there are no meaningful limits.

Nesting of terms

Most built-in predicates that process Prolog terms create an explicitly managed stack and perform optimization for processing the last argument of a term. This implies they can process deeply nested terms at constant and low usage of the C stack, and the system raises a resource error if no more stack can be allocated. Currently only read/1 and write/1 (and all variations thereof) still use the C stack and may cause the system to crash in an uncontrolled way (i.e., not mapped to a Prolog exception that can be caught).

Integers

On most systems SWI-Prolog is compiled with support for unbounded integers by means of the GNU GMP library. In practice this means that integers are bound by the global stack size. Too large integers cause a resource_error. On systems that lack GMP, integers are 64-bit on 32- as well as 64-bit machines.

Integers up to the value of the max_tagged_integer Prolog flag are represented more efficiently on the stack. For integers that appear in clauses, the value (below max_tagged_integer or not) has little impact on the size of the clause.

Floating point numbers

Floating point numbers are represented as C-native double precision floats, 64-bit IEEE on most machines.

2.20.3 Reserved Names

The boot compiler (see -b option) does not support the module system. As large parts of the system are written in Prolog itself we need some way to avoid name clashes with the user's predicates, database keys, etc. Like Edinburgh C-Prolog Pereira, 1986 all predicates, database keys, etc., that should be hidden from the user start with a dollar ($) sign.