Operating System Interaction

This section describes the various system parameters required to run Prolog.

There is normally no need for you to seek any special privileges or quotas in order to run Prolog. Prolog will automatically expand its space up to the total amount of virtual space you are allowed. If it should run out of space, Prolog will raise a resource error.

This may happen because of an infinite recursion in your program, or it may be that your program really needs more space than is available. Under UNIX, if you are using the C shell (csh), you can find out how much space is available by means of the csh command limit. The command

     % limit
     

will list a number of limits of which the relevant one is datasize. This number is the number of kilobytes available to Prolog for its data areas. You can reduce this limit by typing, for example,

     % limit datasize 2000
     

The main reason that you might want to reduce the limit is that some systems allow the allocation of more virtual memory than there is swap space available, and then to crash. You can run quite large programs with a datasize of 2000 kilobytes.

UNIX Caveat:

On some UNIX systems, the specified datasize (program) limit (see limit(csh) and getrlimit(3)) can be grossly higher then the maximum break that a process can set. This is because the setting of the break is dependent upon the amount of swap space available. Since all processes share the same swap space, the space available to any one process is based on the space usage of all other processes running on the machine. Therefore, one process that has set a large program break may prevent another process from doing the same, if both are running simultaneously.

The Quintus Prolog memory manager makes calculations based upon the specified datasize limit, since the actual limit cannot be determined except by experimentation, and even then the limit changes over time. Better memory management will result when the specified datasize limit is close to the actual limit.

The default behavior of Prolog is tuned to be optimal for a large class of programs. If the programmers need greater control of the way Prolog grows and frees memory, they can set environment variables, the documentation for which follow. Note that the default values for these variables should satisfy almost all programs and you really do not need to set these variables at all. The values for these variables are entered in bytes, but may be followed by K or M meaning kilobytes or megabytes respectively.

PROLOGINITSIZE

Controls the size of Prolog's initial memory allocation. Can be set to a sufficiently large size to allow the Prolog application to execute without needing to expand. This must be done before Prolog is invoked.

By default, the value is the minimum memory required for Prolog to start up. In addition, the value is constrained to be at least that amount, regardless of the user setting.

PROLOGMAXSIZE

Can be used to place a limit on the amount of data space that a given Prolog process will use.

The csh command limit can also be used to set the amount of data space that can be used by the the current shell and all processes within it.

By default, the value is effectively infinity, which is to say that Prolog's expansion will only be limited by the space that the shell is able to provide it.

PROLOGINCSIZE

Can be used to control the amount of space Prolog asks the operating system for in any given memory expansion.

By default, the value is the minimum amount of memory that will allow Prolog to expand one of its data areas, by kilobytes. In addition, the value is constrained to be at least that amount, regardless of the user setting.

PROLOGKEEPSIZE

Can be used to control the amount of space Prolog retains after performing some computation. By default, Prolog gets memory from the operating system as the user program executes and returns all the free memory back to the operating system when the user program does not need any more. If the programmer knows that her program once it has grown to a certain size is likely to need as much memory for future computations, then she can advise Prolog not to return all the free memory back to the operating system by setting the value to K. Once Prolog grows to K bytes, it will always keep at least K bytes around. Only memory that was allocated above and beyond K bytes is returned to the OS.

PROLOGLOCALMIN

Can be used to control the amount of space Prolog reserves for the local stack. The purpose of the local stack is described in detail in ref-mgc. The default value is 64Kb.

PROLOGGLOBALMIN

Can be used to control the amount of space Prolog reserves for the global stack. The purpose of the global stack is described in detail in ref-mgc. The default value is 64Kb.