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add GC.safepoint() for compute-bound threads #33092

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Sep 4, 2019
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add GC.safepoint() for compute-bound threads #33092

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15 changes: 15 additions & 0 deletions base/gcutils.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -94,4 +94,19 @@ macro preserve(args...)
end)
end

"""
GC.safepoint()

Inserts a point in the program where garbage collection may run.
This can be useful in rare cases in multi-threaded programs where some threads
are allocating memory (and hence may need to run GC) but other threads are doing
only simple operations (no allocation, task switches, or I/O).
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@KristofferC KristofferC Aug 27, 2019

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Ok, but where would you put this call?

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@StefanKarpinski StefanKarpinski Aug 27, 2019

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An example would be pretty helpful here.

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@mbauman mbauman Aug 27, 2019 •
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Naive question, and maybe this isn't the place to put this information, but why does the compute thread need to have a safepoint if it's not allocating any objects? If the other threads are allocating then they're hitting safepoints... so if the GC determines it needs to collect on one thread does it block and wait for all other threads to similarly block at safepoints before collecting?

Edit: so would that mean you'd want to structure it in the compute loop to run every N milliseconds, where N is near your maximum acceptable GC latency of the other threads and/or on the same order as the average time-to-safepoint of the other threads?

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@JeffBezanson JeffBezanson Aug 27, 2019

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if the GC determines it needs to collect on one thread does it block and wait for all other threads to similarly block at safepoints before collecting

Yes.

Running it every N milliseconds would be nice, but pretty fancy. If you can tolerate a bit of slowdown you can just throw it in a loop anywhere. It's only a single load instruction so it's not horribly expensive. I also wouldn't advise using it at all unless the computation takes a really long time.

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@mbauman mbauman Aug 27, 2019

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Right — I wasn't imaging a timer, but rather giving the docs reader some sort of rule of thumb to allow them to do some reasoning about a rough number of inner loop iterations to run before hitting a safepoint. Of course, you also wouldn't want it in the lowest of loops as it'd break vectorizaton.

Calling this function periodically in non-allocating threads allows garbage
collection to run.

!!! compat "Julia 1.4"
This function is available as of Julia 1.4.
"""
safepoint() = ccall(:jl_gc_safepoint, Cvoid, ())

end # module GC
1 change: 1 addition & 0 deletions doc/src/base/base.md
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Expand Up @@ -395,6 +395,7 @@ Base.functionloc(::Method)
Base.GC.gc
Base.GC.enable
Base.GC.@preserve
Base.GC.safepoint
Meta.lower
Meta.@lower
Meta.parse(::AbstractString, ::Int)
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