Let GCs run while the watchdog sleeps (Julia 1.12+)

[davidanthoff]

Summary

On Julia 1.12 and later, the test server's watchdog thread now sleeps in a GC-safe region. Stop-the-world collections in a test process no longer have to wait for it to wake up.

Why

_watchdog_loop sleeps with Libc.systemsleep(0.05), which is a plain ccall. The thread therefore stays GC-unsafe for the whole sleep. Every collection in the test process, including the frequent young ones during an item, waits until the watchdog wakes and reaches its GC.safepoint().

• How much: 25–50 ms per GC on Windows and 50–190 ms per GC on GitHub macOS runners.
• Why nobody saw it: none of this time shows up in the GC time reported for an item, because total_time excludes time-to-safepoint.

Changes

• New _watchdog_sleep(seconds):
  • Julia ≥ 1.12: @ccall gc_safe=true to usleep on Unix and to libuv's uv_sleep on Windows.
  • Older Julia: Libc.systemsleep, unchanged. @static ensures older versions never expand the new form. The file loads and sleeps correctly on Julia 1.0 through 1.13.
• Why uv_sleep on Windows: @ccall can't express stdcall, and calling Win32 Sleep as cdecl would corrupt the stack on 32-bit Windows. uv_sleep is a cdecl function exported by the Julia runtime, and on Windows it is exactly Sleep(msec). It doesn't touch the event loop.
• The GC.safepoint() in the loop stays, because older Julia still depends on it. Comments are updated.
• CHANGELOG entry under Fixed.

Measurements

Base.gc_num().total_time_to_safepoint per GC.gc(true), averaged over 5 collections. Setup: --threads=1,2, a 200 MB heap, the real watchdog.jl loop running.

Julia	Before	After
1.11.9 x64	23.8 ms	unchanged (fallback path)
1.12.7 x64	22.3 ms	0.0 ms
1.13.0 x64	23.7 ms	0.0 ms
1.12.7 x86 (32-bit Windows)	23.4 ms	0.0 ms

The Unix usleep path was not run locally, since I only have Windows. It's the same call Libc.systemsleep makes, with gc_safe=true added, and CI runs it on Linux and macOS.

Testing

test_worker_lifecycle.jl, test_timeout.jl and test_shutdown.jl pass on Julia 1.12 (89/89). They include "GC between test items does not deadlock the test process" and the hang-diagnostics timeout item. The Julia 1.0/1.6/1.11/1.12 platform items also pass.

Related

Found during the arm64 GC investigation, alongside #121 and #122.

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