Overhead calculation -------------------- The overhead can be shown in two columns as 'Children' and 'Self' when perf collects callchains. The 'self' overhead is simply calculated by adding all period values of the entry - usually a function (symbol). This is the value that perf shows traditionally and sum of all the 'self' overhead values should be 100%. The 'children' overhead is calculated by adding all period values of the child functions so that it can show the total overhead of the higher level functions even if they don't directly execute much. 'Children' here means functions that are called from another (parent) function. It might be confusing that the sum of all the 'children' overhead values exceeds 100% since each of them is already an accumulation of 'self' overhead of its child functions. But with this enabled, users can find which function has the most overhead even if samples are spread over the children. Consider the following example; there are three functions like below. ----------------------- void foo(void) { /* do something */ } void bar(void) { /* do something */ foo(); } int main(void) { bar() return 0; } ----------------------- In this case 'foo' is a child of 'bar', and 'bar' is an immediate child of 'main' so 'foo' also is a child of 'main'. In other words, 'main' is a parent of 'foo' and 'bar', and 'bar' is a parent of 'foo'. Suppose all samples are recorded in 'foo' and 'bar' only. When it's recorded with callchains the output will show something like below in the usual (self-overhead-only) output of perf report: ---------------------------------- Overhead Symbol ........ ..................... 60.00% foo | --- foo bar main __libc_start_main 40.00% bar | --- bar main __libc_start_main ---------------------------------- When the --children option is enabled, the 'self' overhead values of child functions (i.e. 'foo' and 'bar') are added to the parents to calculate the 'children' overhead. In this case the report could be displayed as: ------------------------------------------- Children Self Symbol ........ ........ .................... 100.00% 0.00% __libc_start_main | --- __libc_start_main 100.00% 0.00% main | --- main __libc_start_main 100.00% 40.00% bar | --- bar main __libc_start_main 60.00% 60.00% foo | --- foo bar main __libc_start_main ------------------------------------------- In the above output, the 'self' overhead of 'foo' (60%) was add to the 'children' overhead of 'bar', 'main' and '\_\_libc_start_main'. Likewise, the 'self' overhead of 'bar' (40%) was added to the 'children' overhead of 'main' and '\_\_libc_start_main'. So '\_\_libc_start_main' and 'main' are shown first since they have same (100%) 'children' overhead (even though they have zero 'self' overhead) and they are the parents of 'foo' and 'bar'. Since v3.16 the 'children' overhead is shown by default and the output is sorted by its values. The 'children' overhead is disabled by specifying --no-children option on the command line or by adding 'report.children = false' or 'top.children = false' in the perf config file. 123dbce14c17eb9ced229e488002735'>x86
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