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