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This tunable namespace affects the behaviour of the gmon profiler. gmon is a component of the GNU C Library which is normally used in conjunction with gprof.
When GCC compiles a program with the -pg option, it instruments
the program with calls to the mcount function, to record the
program’s call graph. At program startup, a memory buffer is allocated
to store this call graph; the size of the buffer is calculated using a
heuristic based on code size. If during execution, the buffer is found
to be too small, profiling will be aborted and no gmon.out file
will be produced. In that case, you will see the following message
printed to standard error:
mcount: call graph buffer size limit exceeded, gmon.out will not be generated
Most of the symbols discussed in this section are defined in the header
sys/gmon.h. However, some symbols (for example mcount)
are not defined in any header file, since they are only intended to be
called from code generated by the compiler.
The heuristic for sizing the call graph buffer is known to be
insufficient for small programs; hence, the calculated value is clamped
to be at least a minimum size. The default minimum (in units of
call graph entries, struct tostruct), is given by the macro
MINARCS. If you have some program with an unusually complex
call graph, for which the heuristic fails to allocate enough space,
you can use this tunable to increase the minimum to a larger value.
To prevent excessive memory consumption when profiling very large
programs, the call graph buffer is allowed to have a maximum of
MAXARCS entries. For some very large programs, the default
value of MAXARCS defined in sys/gmon.h is too small; in
that case, you can use this tunable to increase it.
Note the value of the maxarcs tunable must be greater or equal
to that of the minarcs tunable; if this constraint is violated,
a warning will printed to standard error at program startup, and
the minarcs value will be used as the maximum as well.
Setting either tunable too high may result in a call graph buffer whose size exceeds the available memory; in that case, an out of memory error will be printed at program startup, the profiler will be disabled, and no gmon.out file will be generated.
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