| GNU gprof: Cycles |
|---|
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The graph may be complicated by the presence of cycles of
recursion in the call graph. A cycle exists if a function calls
another function that (directly or indirectly) calls (or appears to
call) the original function. For example: if a calls b,
and b calls a, then a and b form a cycle.
Whenever there are call paths both ways between a pair of functions, they
belong to the same cycle. If a and b call each other and
b and c call each other, all three make one cycle. Note that
even if b only calls a if it was not called from a,
gprof cannot determine this, so a and b are still
considered a cycle.
The cycles are numbered with consecutive integers. When a function belongs to a cycle, each time the function name appears in the call graph it is followed by ‘<cycle number>’.
The reason cycles matter is that they make the time values in the call
graph paradoxical. The “time spent in children” of a should
include the time spent in its subroutine b and in b’s
subroutines—but one of b’s subroutines is a! How much of
a’s time should be included in the children of a, when
a is indirectly recursive?
The way gprof resolves this paradox is by creating a single entry
for the cycle as a whole. The primary line of this entry describes the
total time spent directly in the functions of the cycle. The
“subroutines” of the cycle are the individual functions of the cycle, and
all other functions that were called directly by them. The “callers” of
the cycle are the functions, outside the cycle, that called functions in
the cycle.
Here is an example portion of a call graph which shows a cycle containing
functions a and b. The cycle was entered by a call to
a from main; both a and b called c.
index % time self children called name
----------------------------------------
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
----------------------------------------
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
----------------------------------------
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
----------------------------------------
(The entire call graph for this program contains in addition an entry for
main, which calls a, and an entry for c, with callers
a and b.)
index % time self children called name
<spontaneous>
[1] 100.00 0 1.93 0 start [1]
0.16 1.77 1/1 main [2]
----------------------------------------
0.16 1.77 1/1 start [1]
[2] 100.00 0.16 1.77 1 main [2]
1.77 0 1/1 a <cycle 1> [5]
----------------------------------------
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
0 0 6/6 c [6]
----------------------------------------
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
----------------------------------------
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
----------------------------------------
0 0 3/6 b <cycle 1> [4]
0 0 3/6 a <cycle 1> [5]
[6] 0.00 0 0 6 c [6]
----------------------------------------
The self field of the cycle’s primary line is the total time
spent in all the functions of the cycle. It equals the sum of the
self fields for the individual functions in the cycle, found
in the entry in the subroutine lines for these functions.
The children fields of the cycle’s primary line and subroutine lines
count only subroutines outside the cycle. Even though a calls
b, the time spent in those calls to b is not counted in
a’s children time. Thus, we do not encounter the problem of
what to do when the time in those calls to b includes indirect
recursive calls back to a.
The children field of a caller-line in the cycle’s entry estimates
the amount of time spent in the whole cycle, and its other
subroutines, on the times when that caller called a function in the cycle.
The called field in the primary line for the cycle has two numbers:
first, the number of times functions in the cycle were called by functions
outside the cycle; second, the number of times they were called by
functions in the cycle (including times when a function in the cycle calls
itself). This is a generalization of the usual split into non-recursive and
recursive calls.
The called field of a subroutine-line for a cycle member in the
cycle’s entry says how many time that function was called from functions in
the cycle. The total of all these is the second number in the primary line’s
called field.
In the individual entry for a function in a cycle, the other functions in
the same cycle can appear as subroutines and as callers. These lines show
how many times each function in the cycle called or was called from each other
function in the cycle. The self and children fields in these
lines are blank because of the difficulty of defining meanings for them
when recursion is going on.
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