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ChuckBuilds
73fff8d2d5
Review follow-up. The range check accepted 1, 3 and 4, so a change to 4 -- which hands most of the resident saving back -- passed a test whose whole purpose is to notice that. Pinned to the value the unit ships, in one named constant. Raising it is still a legitimate response to a frame-time regression, but it should be a visible edit here rather than silent drift, and the failure message says so. Mutation-checked: changing the unit to 4 now fails.
96 lines
3.8 KiB
Python
96 lines
3.8 KiB
Python
"""The display unit must cap glibc's malloc arenas.
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glibc hands each allocating thread its own malloc arena, up to 8 x CPU count,
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and an arena that has grown is never returned to the OS. This process runs
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threads for the render loop, the update workers and the background fetchers, so
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on a 3-core Pi the ceiling is 24 arenas.
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Measured on a live rig, 2.5 hours in:
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RSS 1030 MB
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Private_Dirty 988 MB
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anonymous mappings > 10 MB 23 (ceiling is 8 x 3 = 24)
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largest few 104, 79, 66, 63, 63 MB, on 64 MB-aligned addresses
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against live data that accounts for perhaps 15 MB -- the widest scroll strip
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observed was 35,746 x 64, about 7 MB as RGB and the same again for its numpy
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mirror. Repeated sampling showed RSS flat between 990 and 1030 MB rather than
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climbing, so this is arena bloat rather than a leak: memory Python has freed
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but glibc is holding per-arena.
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The device had 59 MB free at the time.
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Capping the arena count trades a little allocator concurrency for that resident
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memory. The render loop is latency-sensitive, so if p99 frame time regresses the
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right response is to raise this rather than remove it.
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"""
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import re
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from pathlib import Path
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import pytest
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UNIT = (Path(__file__).resolve().parent.parent / "systemd" / "ledmatrix.service")
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#: The value the unit is expected to carry. 2 is the usual choice for a
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#: threaded Python process; 1-4 all keep some of the saving, but only one of
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#: them is what this project ships.
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EXPECTED_ARENA_MAX = 2
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def _environment(unit_text):
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return dict(
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line.split("=", 2)[1:3] if line.count("=") >= 2 else (line.split("=", 1)[1], "")
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for line in unit_text.splitlines()
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if line.startswith("Environment=")
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)
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def test_the_unit_exists():
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assert UNIT.is_file(), f"{UNIT} is missing"
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def test_malloc_arena_max_is_capped():
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env = _environment(UNIT.read_text(encoding="utf-8"))
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assert "MALLOC_ARENA_MAX" in env, (
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"the display unit does not cap glibc arenas; on a 3-core Pi the default "
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"ceiling is 24 and a measured rig held 23 of them, 920 MB"
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)
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value = int(env["MALLOC_ARENA_MAX"])
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# Pinned, not a range. A range let a change to 4 -- which hands most of the
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# saving back -- pass unnoticed, which was the point of the finding that
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# prompted this. Raising it is a legitimate response to a frame-time
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# regression, but it should be a visible edit here rather than a silent
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# drift, so the number lives in one place and changing it shows up in
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# review.
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assert value == EXPECTED_ARENA_MAX, (
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f"MALLOC_ARENA_MAX={value}, expected {EXPECTED_ARENA_MAX}. If this was "
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"raised deliberately because frame times regressed, update "
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"EXPECTED_ARENA_MAX here and say so in the commit."
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)
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def test_the_reason_is_recorded_next_to_it():
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"""A bare tuning knob invites removal by whoever meets it next."""
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text = UNIT.read_text(encoding="utf-8")
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index = text.index("Environment=MALLOC_ARENA_MAX")
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preamble = text[:index].splitlines()[-12:]
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comment = "\n".join(line for line in preamble if line.startswith("#"))
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assert "arena" in comment.lower(), "no explanation precedes the setting"
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assert re.search(r"\d", comment), (
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"the explanation cites no measurement, so a reader cannot tell whether "
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"it still applies to their hardware"
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)
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@pytest.mark.parametrize("unit", ["ledmatrix.service"])
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def test_the_unit_still_parses_as_ini(unit):
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"""systemd will refuse a malformed unit, and the panel stays dark."""
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import configparser
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path = UNIT.parent / unit
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parser = configparser.ConfigParser(strict=False)
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# systemd allows repeated keys; ConfigParser needs them merged, not rejected.
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parser.read_string(path.read_text(encoding="utf-8"))
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assert parser.has_section("Service")
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assert parser.has_option("Service", "ExecStart")
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