1 Overhead calculation
2 --------------------
3 The overhead can be shown in two columns as 'C
4 perf collects callchains. The 'self' overhead
5 adding all period values of the entry - usuall
6 This is the value that perf shows traditionall
7 'self' overhead values should be 100%.
8
9 The 'children' overhead is calculated by addin
10 the child functions so that it can show the to
11 higher level functions even if they don't dire
12 'Children' here means functions that are calle
13 function.
14
15 It might be confusing that the sum of all the
16 values exceeds 100% since each of them is alre
17 'self' overhead of its child functions. But w
18 can find which function has the most overhead
19 spread over the children.
20
21 Consider the following example; there are thre
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 'b
40 child of 'main' so 'foo' also is a child of 'm
41 'main' is a parent of 'foo' and 'bar', and 'ba
42
43 Suppose all samples are recorded in 'foo' and
44 recorded with callchains the output will show
45 in the usual (self-overhead-only) output of pe
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 'se
65 child functions (i.e. 'foo' and 'bar') are add
66 calculate the 'children' overhead. In this ca
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 'f
96 'children' overhead of 'bar', 'main' and '\_\_
97 Likewise, the 'self' overhead of 'bar' (40%) w
98 'children' overhead of 'main' and '\_\_libc_st
99
100 So '\_\_libc_start_main' and 'main' are shown
101 same (100%) 'children' overhead (even though t
102 overhead) and they are the parents of 'foo' an
103
104 Since v3.16 the 'children' overhead is shown b
105 is sorted by its values. The 'children' overhe
106 specifying --no-children option on the command
107 'report.children = false' or 'top.children = f
108 file.
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