Tachyon’s `--diff-flamegraph` scales times in baseline run to duration of current run

Bug report

Bug description:

As discussed with @pablogsal during EuroPython sprints:

Using this simple example script (flamegraph_demo.py):

import time

def main():
    f_1()
    f_2()

def f_1():
    time.sleep(0.5)  # change this to 1.5 in the second run

def f_2():
    time.sleep(1)

main()

python -m profiling.sampling run --binary -o baseline.bin flamegraph_demo.py
vim flamegraph_demo.py  # edit f_1 to sleep for 1.5 s
python -m profiling.sampling run --diff-flamegraph baseline.bin -o diff.html flamegraph_demo.py

generates this diff flamegraph:

f_2 is unchanged between both runs, so the “Baseline Self” time should be equal to the “Current Self” and equal to 1 s (that it displays “1 ms” is covered in #154059); however, the baseline for all functions gets scaled with the ratio of total time in this run over total time of the previous run (in this case: 2.5 s / 1.5 s = 1.667).

I can imagine scenarios where this scaling is useful (e.g. when the baseline is generated on a different machine). However, in the common case of running baseline and diff on the same machine, this scaling means that a lot of unchanged functions get colour-coded, which distracts from the few functions that experienced a meaningful change.

CPython versions tested on:

3.15

Operating systems tested on:

macOS

Linked PRs

• gh-154082

Start with the profiling.sampling entry point and reproduce the commands using flamegraph_demo.py, comparing baseline.bin with diff.html. Trace how --diff-flamegraph scales baseline times and verify that unchanged f_2 shows equal 1 s baseline and current self times without unnecessary color changes.