profil(2) profil(2)
NAME
profil - execution time profile
SYNOPSIS
#include <unistd.h>
void profil(unsigned short *buff, sizet bufsiz, int offset, unsigned
scale);
DESCRIPTION
profil provides CPU-use statistics by profiling the amount of CPU
time expended by a program. profil generates the statistics by
creating an execution histogram for a current process. The histogram
is defined for a specific region of program code to be profiled, and
the identified region is logically broken up into a set of equal size
subdivisions, each of which corresponds to a count in the histogram.
With each clock tick, the current subdivision is identified and its
corresponding histogram count is incremented. These counts establish
a relative measure of how much time is being spent in each code
subdivision. The resulting histogram counts for a profiled region
can be used to identify those functions that consume a
disproportionately high percentage of CPU time.
buff is a buffer of bufsiz bytes in which the histogram counts are
stored in an array of unsigned short int.
offset, scale, and bufsiz specify the region to be profiled.
offset is effectively the start address of the region to be profiled.
scale, broadly speaking, is a contraction factor that indicates how
much smaller the histogram buffer is than the region to be profiled.
More precisely, scale is interpreted as an unsigned 16-bit fixed-
point fraction with the decimal point implied on the left. Its value
is the reciprocal of the number of bytes in a subdivision, per byte
of histogram buffer. Since there are two bytes per histogram
counter, the effective ratio of subdivision bytes per counter is one
half the scale.
Several observations can be made:
- the maximal value of scale, 0xffff (approximately 1), maps
subdivisions 2 bytes long to each counter.
- the minimum value of scale (for which profiling is
performed), 0x0002 (1/32,768), maps subdivision 65,536 bytes
long to each counter.
- the default value of scale (currently used by cc -qp),
0x4000, maps subdivisions 8 bytes long to each counter.
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profil(2) profil(2)
The values are used within the kernel as follows: when the process
is interrupted for a clock tick, the value of offset is subtracted
from the current value of the program counter (pc), and the remainder
is multiplied by scale to derive a result. That result is used as an
index into the histogram array to locate the cell to be incremented.
Therefore, the cell count represents the number of times that the
process was executing code in the subdivision associated with that
cell when the process was interrupted.
scale can be computed as (RATIO * 0200000L), where RATIO is the
desired ratio of bufsiz to profiled region size, and has a value
between 0 and 1. Qualitatively speaking, the closer RATIO is to 1,
the higher the resolution of the profile information.
bufsiz can be computed as (size_of_region_to_be_profiled * RATIO).
SEE ALSO
prof(1), times(2), monitor(3C).
NOTES
Profiling is turned off by giving a scale of 0 or 1, and is rendered
ineffective by giving a bufsiz of 0. Profiling is turned off when an
exec(2) is executed, but remains on in both child and parent
processes after a fork(2). Profiling is turned off if a buff update
would cause a memory fault.
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