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Subsections

Introduction

This chapter may be skipped on a first reading. Its purpose is to give the advanced user a better understanding of how the system uses memory resources. In a high level language like Prolog it is often not obvious for the programmer to see where the system allocates or frees memory.

The sizes of the different memory areas can be queried by means of the predicate statistics/2 and statistics/0 prints a summary of all these data. Here is a sample output:

[eclipse 1]: statistics.

times:                  [0.133333, 0.65, 304.809] seconds
global_stack_used:      724 bytes
global_stack_allocated: 102400 bytes
global_stack_peak:      1576960 bytes
trail_stack_used:       32 bytes
trail_stack_allocated:  16384 bytes
trail_stack_peak:       16384 bytes
control_stack_used:     392 bytes
control_stack_allocated:4096 bytes
control_stack_peak:     4096 bytes
local_stack_used:       288 bytes
local_stack_allocated:  4096 bytes
local_stack_peak:       4096 bytes
code_heap_allocated:    326888 bytes
code_heap_used:         326888 bytes
general_heap_allocated: 291608 bytes
general_heap_used:      278432 bytes
gc_number:              0 
gc_collected:           0 bytes
gc_area:                0 bytes
gc_ratio:               100.0 %
gc_time:                0.0 seconds
dictionary_entries:     2494 
dict_hash_usage:        1498 / 8191 
dict_hash_collisions:   155 / 1498 
dict_gc_number:         0 
dict_gc_time:           0.0 seconds
The used-figures indicate the actual usage at the moment the statistics built-in was called. The allocated value is the amount of memory that is reserved for this area and actually occupied by the ECLiPSe process. The peak value indicates what was the maximum allocated amount during the session. In the following we will discuss the six memory areas mentioned. The gc-figures are described in section 19.2.


The Code Heap

The code heap is used to store compiled Prolog code and external predicates. Consequently its size is increased by the various compile-predicates, the assert-family and by load/1.

Space on the code heap is freed when single clauses (retract) or whole predicates (abolish) are removed from the system. Note that space reclaiming is usually delayed in these cases, since the removed code may still be under execution. Erasing a module also reclaims all the memory occupied by the module's predicates.


The General Heap

The general heap is used to store a variety of data:


The Local Stack

The Local Stack is very similar to the call/return stack in procedural languages. It holds Prolog variables and return addresses. Space on this stack is allocated during execution of a clause and deallocated before the last subgoal is called (due to tail recursion / last call optimisation). This deallocation can not be done when the clause exits nondeterministically (this can be checked with the debugger or the profiling facility). However, if a deallocation has been delayed due to nondeterminism, it is finally done when a cut is executed or when execution fails beyond the allocation point. Hence the ways to limit growth of the local stack are


The Control Stack

The main use of the Control Stack is to store so-called choicepoints. A choicepoint is a description of the system's state at a certain point in execution. It is created when more than one clause of a predicate apply to a given goal. Should the first clause fail, the system will backtrack to the place where the choice was made, the old state will be restored from the choicepoint and the next clause will be tried. Disjunctions (;/2) also create choicepoints.

The only way to reduce Control Stack usage is to avoid unnecessary nondeterminism. This is done by writing deterministic predicates in such a way that they can be recognised by the system. The debugger can help to identify nondeterministic predicates: When it displays an *EXIT port instead of EXIT then the predicate has left a choicepoint behind. In this case it should be checked whether the nondeterminism was intended. If not, the predicate can often be made deterministic by


The Global Stack

The Global Stack holds Prolog structures, lists, strings and long numbers. So the user's selection of data structures is largely responsible for the growth of this stack (cf. 6.4). In coroutining mode, delayed goals also consume space on the Global Stack. It also stores source variable names for terms which were read in with the flag variable_names being on. When this feature is not needed, it should be turned off so that space on the global stack is saved.

The global stack grows while a program creates data structures. It is popped only on failure. ECLiPSe therefore provides a garbage collector for the Global Stack which is called when a certain amount of new space has been consumed. See section 19.2 for how this process can be controlled. Note again that unnecessary nondeterminism reduces the amount of garbage that can be reclaimed and should therefore be avoided.


The Trail Stack

The Trail Stack is used to record information that is needed on backtracking. It is therefore closely related to the Control Stack. Ways to reduce Trail Stack consumption are The Trail Stack is popped on failure and is garbage collected together with the Global Stack.


next up previous index
Next: Garbage collection Up: Memory Organisation And Garbage Previous: Memory Organisation And Garbage   Index

1999-08-06