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cmd/compile/internal/pgo: count only the last two frames as a call edge

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Currently for every CPU profile sample, we apply its weight to all
call edges of the entire call stack. Frames higher up the stack
are unlikely to be repeated calls (e.g. runtime.main calling
main.main). So adding weights to call edges higher up the stack
may be not reflecting the actual call edge weights in the program.
This CL changes it to add weights to only the edge between the
last two frames.

Without a branch profile (e.g. LBR records) this is not perfect,
but seems more reasonable.

Change-Id: I0aee75cc608a152adad41c51120b661a6c542283
Reviewed-on: https://go-review.googlesource.com/c/go/+/450915
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Cherry Mui 2022-11-15 13:37:42 -05:00
parent 3f1bcc58b3
commit 6e0e492e12
1 changed files with 11 additions and 1 deletions
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12
src/cmd/compile/internal/pgo/graph.go
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@ -270,7 +270,17 @@ func newGraph(prof *profile.Profile, o *Options) *Graph {
residual := false
// Group the sample frames, based on a global map.
for i := len(sample.Location) - 1; i >= 0; i-- {
// Count only the last two frames as a call edge. Frames higher up
// the stack are unlikely to be repeated calls (e.g. runtime.main
// calling main.main). So adding weights to call edges higher up
// the stack may be not reflecting the actual call edge weights
// in the program. Without a branch profile this is just an
// approximation.
i := 1
if last := len(sample.Location) - 1; last < i {
i = last
}
for ; i >= 0; i-- {
l := sample.Location[i]
locNodes := locationMap.get(l.ID)
for ni := len(locNodes) - 1; ni >= 0; ni-- {