4.41 Obtaining Runtime Statistics

The predicate statistics/2 is built-in. More high level predicates are available from library library(statistics). See section A.52.

statistics(+Key, -Value)

Unify system statistics determined by Key with Value. The possible keys are given in the table 6. This predicate supports additional keys for compatibility reasons. These keys are described in table 7. CPU time results are based on clock_gettime(), times() or wall time since the process was started (in that order of preference). On Windows GetProcessTimes() is used. Both clock_gettime() and GetProcessTimes() provide a nanosecond resolution interface. The actual resolution depends on the platform.

Starting with version 9.1.9, the cputime and inferences keys include the final value for threads that have been created by the calling thread and has been joined by the calling thread. The new keys self_cputime and self_inferences may be used to get statistics for the calling thread only. Both keys also exist in the single threaded version, where the “self” key always returns the same value as the onle without “self” .

Native keys (times as float in seconds)
agc	Number of atom garbage collections performed
agc_gained	Number of atoms removed
agc_time	Time spent in atom garbage collections
atoms	Total number of defined atoms
atom_space	Bytes used to represent atoms
c_stack	System (C-) stack limit. 0 if not known.
cgc	Number of clause garbage collections performed
cgc_gained	Number of clauses reclaimed
cgc_time	Time spent in clause garbage collections
clauses	Total number of clauses in the program
codes	Total size of (virtual) executable code in words
cputime	(User) CPU time since thread was started in seconds. Includes CPU time in completed child threads. See also self_cputime and process_cputime.
epoch	Time stamp when thread was started
errors	Number of error mesages printed
functors	Total number of defined name/arity pairs
functor_space	Bytes used to represent functors
global	Allocated size of the global stack in bytes
globalused	Number of bytes in use on the global stack
globallimit	Size to which the global stack is allowed to grow
global_shifts	Number of global stack expansions
heapused	Bytes of heap in use by Prolog (0 if not maintained)
inferences	Total number of passes via the call and redo ports since Prolog was started. Includes inferences in child threads. See also self_inferences.
modules	Total number of defined modules
local	Allocated size of the local stack in bytes
local_shifts	Number of local stack expansions
locallimit	Size to which the local stack is allowed to grow
localused	Number of bytes in use on the local stack
table_space_used	Amount of bytes in use by the thread's answer tables
trail	Allocated size of the trail stack in bytes
trail_shifts	Number of trail stack expansions
traillimit	Size to which the trail stack is allowed to grow
trailused	Number of bytes in use on the trail stack
shift_time	Time spent in stack-shifts
self_cputime	(User) CPU time since thread was started in seconds
self_inferences	Total number of passes via the call and redo ports since Prolog was started
stack	Total memory in use for stacks in all threads
predicates	Total number of predicates. This includes predicates that are undefined or not yet resolved.
indexes_created	Number of clause index tables creates.
indexes_destroyed	Number of clause index tables destroyed.
process_epoch	Time stamp when Prolog was started
process_cputime	(User) CPU time since Prolog was started in seconds
thread_cputime	MT-version: Seconds CPU time used by finished threads. The implementation requires non-portable functionality. Currently works on Linux, MacOSX, Windows and probably some more.
threads	MT-version: number of active threads
threads_created	MT-version: number of created threads
engines	MT-version: number of existing engines
engines_created	MT-version: number of created engines
threads_peak	MT-version: highest id handed out. This is a fair but possibly not 100% accurate value for the highest number of threads since the process was created.
warnings	Number of warning mesages printed

Table 6 : Keys for statistics/2. Space is expressed in bytes. Time is expressed in seconds, represented as a floating point number.

Compatibility keys (times in milliseconds)
runtime	[ CPU time, CPU time since last ] (milliseconds, excluding time spent in garbage collection)
system_time	[ System CPU time, System CPU time since last ] (milliseconds)
real_time	[ Wall time, Wall time since last ] (integer seconds. See get_time/1)
walltime	[ Wall time since start, Wall time since last] (milliseconds, SICStus compatibility)
memory	[ Total unshared data, free memory ] (Used is based on ru_idrss from getrusage(). Free is based on RLIMIT_DATA from getrlimit(). Both are reported as zero if the OS lacks support. Free is -1 if getrlimit() is supported but returns infinity.)
stacks	[ global use, local use ]
program	[ heap use, 0 ]
global_stack	[ global use, global free ]
local_stack	[ local use, local free ]
trail	[ trail use, trail free ]
garbage_collection	[ number of GC, bytes gained, time spent, bytes left ] The last column is a SWI-Prolog extension. It contains the sum of the memory left after each collection, which can be divided by the count to find the average working set size after GC. Use [Count, Gained, Time|_] for compatibility.
stack_shifts	[ global shifts, local shifts, time spent ]
atoms	[ number, memory use, 0 ]
atom_garbage_collection	[ number of AGC, bytes gained, time spent ]
clause_garbage_collection	[ number of CGC, clauses gained, time spent ]
core	Same as memory

Table 7 : Compatibility keys for statistics/2. Time is expressed in milliseconds.