4.10.5 Interaction of Garbage Collection and Global Stack Expansion

For most programs, the default settings for the garbage collection parameters should suffice. For programs that have high global stack requirements, the default parameters may result in a higher ratio of garbage collection time to run time. These programs should be given more space in which to run.

The gc_margin is a non-negative integer specifying the desired margin in kilobytes. For example, the default value of 1000 means that the global stack will not be expanded if garbage collection can reclaim at least one megabyte. The advantage of this criterion is that it takes into account both the user’s estimate of the global stack usage and the effectiveness of garbage collecting.

1. Setting the gc_margin higher than the default will cause fewer global stack expansions and garbage collections. However, it will use more space, and garbage collections will be more time-consuming when they do occur.

   Setting the margin too large will cause the global stack to expand so that if it does overflow, then the resulting garbage collection will significantly disrupt normal processing. This will be especially so if much of the global stack is accessible to future computation.

2. Setting the gc_margin lower than the default will use less space, and garbage collections will be less time-consuming. However, it will cause more global stack expansions and garbage collections.

   Setting the margin too small will cause many garbage collections in a small amount of time, so that the ratio of garbage-collecting time to computation time will be abnormally high.

3. Setting the margin correctly will cause the global stack to expand to a size where expansions and garbage collections are infrequent and garbage collections are not too time-consuming, if they occur at all.

The correct value for the gc_margin is dependent upon many factors. Here is a non-prioritized list of some of them:

• The amount of memory available to the Prolog process
• The maximum memory limit imposed on the Prolog process
• The program’s rate of global stack garbage generation
• The program’s rate of global stack non-garbage generation
• The program’s backtracking behavior
• The amount of time needed to collect the generated garbage
• The growth rate of the other Prolog stacks

The algorithm used when the global stack overflows is as follows:

if gc is on and
the global stack has grown at least gc_margin kilobytes
since the last garbage collection then
   garbage collect the global stack
   if less than gc_margin kilobytes are reclaimed then
      try to expand the global stack
   endif
else 
   try to expand the global stack
endif 

The user can use the gc_margin option of prolog_flag/3 to reset the gc_margin (see ref-lps-ove). If a garbage collection reclaims at least the gc_margin kilobytes of global stack space, then the global stack is not expanded after garbage collection completes. Otherwise, the global stack is expanded after garbage collection. This expansion provides space for the future global stack usage that will presumably occur. In addition, no garbage collection occurs if the global stack has grown less than gc_margin kilobytes since the last garbage collection.