1 Overhead calculation
2 --------------------
3 The overhead can be shown in two columns as 'Children' and 'Self' when
4 perf collects callchains. The 'self' overhead is simply calculated by
5 adding all period values of the entry - usually a function (symbol).
6 This is the value that perf shows traditionally and sum of all the
7 'self' overhead values should be 100%.
8
9 The 'children' overhead is calculated by adding all period values of
10 the child functions so that it can show the total overhead of the
11 higher level functions even if they don't directly execute much.
12 'Children' here means functions that are called from another (parent)
13 function.
14
15 It might be confusing that the sum of all the 'children' overhead
16 values exceeds 100% since each of them is already an accumulation of
17 'self' overhead of its child functions. But with this enabled, users
18 can find which function has the most overhead even if samples are
19 spread over the children.
20
21 Consider the following example; there are three functions like below.
22
23 -----------------------
24 void foo(void) {
25 /* do something */
26 }
27
28 void bar(void) {
29 /* do something */
30 foo();
31 }
32
33 int main(void) {
34 bar()
35 return 0;
36 }
37 -----------------------
38
39 In this case 'foo' is a child of 'bar', and 'bar' is an immediate
40 child of 'main' so 'foo' also is a child of 'main'. In other words,
41 'main' is a parent of 'foo' and 'bar', and 'bar' is a parent of 'foo'.
42
43 Suppose all samples are recorded in 'foo' and 'bar' only. When it's
44 recorded with callchains the output will show something like below
45 in the usual (self-overhead-only) output of perf report:
46
47 ----------------------------------
48 Overhead Symbol
49 ........ .....................
50 60.00% foo
51 |
52 --- foo
53 bar
54 main
55 __libc_start_main
56
57 40.00% bar
58 |
59 --- bar
60 main
61 __libc_start_main
62 ----------------------------------
63
64 When the --children option is enabled, the 'self' overhead values of
65 child functions (i.e. 'foo' and 'bar') are added to the parents to
66 calculate the 'children' overhead. In this case the report could be
67 displayed as:
68
69 -------------------------------------------
70 Children Self Symbol
71 ........ ........ ....................
72 100.00% 0.00% __libc_start_main
73 |
74 --- __libc_start_main
75
76 100.00% 0.00% main
77 |
78 --- main
79 __libc_start_main
80
81 100.00% 40.00% bar
82 |
83 --- bar
84 main
85 __libc_start_main
86
87 60.00% 60.00% foo
88 |
89 --- foo
90 bar
91 main
92 __libc_start_main
93 -------------------------------------------
94
95 In the above output, the 'self' overhead of 'foo' (60%) was add to the
96 'children' overhead of 'bar', 'main' and '\_\_libc_start_main'.
97 Likewise, the 'self' overhead of 'bar' (40%) was added to the
98 'children' overhead of 'main' and '\_\_libc_start_main'.
99
100 So '\_\_libc_start_main' and 'main' are shown first since they have
101 same (100%) 'children' overhead (even though they have zero 'self'
102 overhead) and they are the parents of 'foo' and 'bar'.
103
104 Since v3.16 the 'children' overhead is shown by default and the output
105 is sorted by its values. The 'children' overhead is disabled by
106 specifying --no-children option on the command line or by adding
107 'report.children = false' or 'top.children = false' in the perf config
108 file.
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