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