tree 4d3de3197e1de614bb6ec5bc00489c30210db8e9
parent c7ffaf80713afbe5a4f4d95a605c0a98084374de
author Matt Stark <msta@google.com> 1787202561 +1000
committer gn-scoped@luci-project-accounts.iam.gserviceaccount.com <gn-scoped@luci-project-accounts.iam.gserviceaccount.com> 1787714942 -0700

Create a trace comparison tool to evaluate the performance impact of a change.

On many occasions, it's useful to understand the performance impact of a
change to GN. In particular, when investigating changes like making ninja
files per directory instead of per target, overall build times can be noisy
and hard to measure reliably.

`tools/compare_traces.py` compares Chrome trace logs from `gn gen --tracelog`
runs to isolate the performance impact on specific build files and phases.
By averaging multiple runs and filtering out noise, it can show when a change
reliably affects a specific part of the build even if the overall wall-clock
difference is small.

Example - I modified gn to sleep for 5 seconds while loading
//chrome/browser/BUILD.gn, then got the following results:

$ tools/compare_traces.py --before before_1.trace before_2.trace \
  before_3.trace --after after_1.trace after_2.trace after_3.trace
======================================================================================
 GN PERFORMANCE IMPACT REPORT
======================================================================================
Samples:  Before: 3 run(s) | After: 3 run(s)

EXECUTIVE VERDICT
--------------------------------------------------------------------------------------
  ⚠️  REGRESSION DETECTED: Slower by +2,420 ms (+67.6%)

  • Wall-clock / run:  Before:    3,577 ± 365 ms  ->  After:     5,997 ± 17 ms
  • Total CPU / run:   Before: 64,738 ± 4,101 ms  ->  After: 76,680 ± 1,848 ms
  • Events / run:      Before:           152,059  ->  After:           152,059  (Δ +0)

AFFECTED GN PHASES (Ranked by impact)
--------------------------------------------------------------------------------------
 Delta (ms)  Delta (%)      Before       After  Name
--------------------------------------------------------------------------------------
  +6,167 ms    +31.5%   19,584 ms   25,752 ms  BUILD.gn Execution
     +26 ms     +4.1%      634 ms      660 ms  External Scripts

AFFECTED SUBSYSTEMS (Ranked by impact)
--------------------------------------------------------------------------------------
 Delta (ms)  Delta (%)      Before       After  Name
--------------------------------------------------------------------------------------
     -22 ms    -65.8%       33 ms       11 ms  //remoting/base

AFFECTED BUILD FILES (Ranked by impact)
--------------------------------------------------------------------------------------
 Delta (ms)  Delta (%)      Before       After  Name
--------------------------------------------------------------------------------------
  +4,940 ms  +7093.8%       70 ms    5,009 ms  //chrome/browser/BUILD.gn
     +25 ms    +19.8%      124 ms      149 ms  //chrome/test/BUILD.gn
     +24 ms    +84.1%       28 ms       52 ms  //components/omnibox/browser/BUILD.gn
     -22 ms    -68.5%       32 ms       10 ms  //remoting/base/BUILD.gn
     -21 ms    -69.8%       30 ms        9 ms  //ui/gl/mojom/BUILD.gn
     -20 ms    -66.8%       30 ms       10 ms  //remoting/protocol/BUILD.gn
     +10 ms    +31.7%       30 ms       40 ms  //extensions/browser/BUILD.gn
     -12 ms    -37.8%       32 ms       20 ms  //ui/accessibility/mojom/BUILD.gn

Change-Id: Ie31079d5836559d850d61692021b1e346a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/25680
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>