xref: /linux/tools/perf/Documentation/callchain-overhead-calculation.txt (revision c532de5a67a70f8533d495f8f2aaa9a0491c3ad0)
1Overhead calculation
2--------------------
3The overhead can be shown in two columns as 'Children' and 'Self' when
4perf collects callchains.  The 'self' overhead is simply calculated by
5adding all period values of the entry - usually a function (symbol).
6This is the value that perf shows traditionally and sum of all the
7'self' overhead values should be 100%.
8
9The 'children' overhead is calculated by adding all period values of
10the child functions so that it can show the total overhead of the
11higher level functions even if they don't directly execute much.
12'Children' here means functions that are called from another (parent)
13function.
14
15It might be confusing that the sum of all the 'children' overhead
16values exceeds 100% since each of them is already an accumulation of
17'self' overhead of its child functions.  But with this enabled, users
18can find which function has the most overhead even if samples are
19spread over the children.
20
21Consider the following example; there are three functions like below.
22
23-----------------------
24void foo(void) {
25    /* do something */
26}
27
28void bar(void) {
29    /* do something */
30    foo();
31}
32
33int main(void) {
34    bar()
35    return 0;
36}
37-----------------------
38
39In this case 'foo' is a child of 'bar', and 'bar' is an immediate
40child 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
43Suppose all samples are recorded in 'foo' and 'bar' only.  When it's
44recorded with callchains the output will show something like below
45in the usual (self-overhead-only) output of perf report:
46
47----------------------------------
48Overhead  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
64When the --children option is enabled, the 'self' overhead values of
65child functions (i.e. 'foo' and 'bar') are added to the parents to
66calculate the 'children' overhead.  In this case the report could be
67displayed as:
68
69-------------------------------------------
70Children      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
95In the above output, the 'self' overhead of 'foo' (60%) was add to the
96'children' overhead of 'bar', 'main' and '\_\_libc_start_main'.
97Likewise, the 'self' overhead of 'bar' (40%) was added to the
98'children' overhead of 'main' and '\_\_libc_start_main'.
99
100So '\_\_libc_start_main' and 'main' are shown first since they have
101same (100%) 'children' overhead (even though they have zero 'self'
102overhead) and they are the parents of 'foo' and 'bar'.
103
104Since v3.16 the 'children' overhead is shown by default and the output
105is sorted by its values. The 'children' overhead is disabled by
106specifying --no-children option on the command line or by adding
107'report.children = false' or 'top.children = false' in the perf config
108file.
109