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Run GC on multiple threads#48600

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vchuravy merged 1 commit intoJuliaLang:masterfrom
d-netto:dcn/gcthreads
Apr 28, 2023
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Run GC on multiple threads#48600
vchuravy merged 1 commit intoJuliaLang:masterfrom
d-netto:dcn/gcthreads

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@d-netto d-netto commented Feb 9, 2023 •
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Summary

This PR parallelizes the GC mark-loop by introducing GC threads into the Julia runtime and by implementing work-stealing to dynamically balance the amount of work each thread performs in the GC mark-loop.

Implementation

Following #47292, each thread running the GC mark-loop manages two work-queues: one queue stores pointers to Julia objects that need to be scanned, and another queue (chunk queue) stores iterator states corresponding to suffixes of large arrays that need to be scanned.

Both the pointer and chunk queues are lock-free and are based on the work of Chase-Lev and Le et. al (see papers referenced in work-stealing-queue.h).

Results

These are speedups in mark-time for a tweaked (JuliaCI/GCBenchmarks#61) version of the rb_tree.jl benchmark from GCBenchmarks. This representative benchmark uses a single mutator thread (we scale the number of GC threads in the plots below). For more benchmarks, see @vchuravy's comment below.

Machine

  • 262 GB RAM
  • ARM Neoverse-N1 80-Core Processor

Speedups

plot

@d-netto d-netto requested a review from vtjnash February 9, 2023 04:40
@d-netto d-netto added the GC Garbage collector label Feb 9, 2023
@d-netto d-netto requested a review from vchuravy February 9, 2023 04:41
@vchuravy vchuravy marked this pull request as draft February 10, 2023 16:05
@vchuravy vchuravy changed the title WIP: GC threads GC threads Feb 10, 2023
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vchuravy commented Feb 10, 2023

I assume this depends on #48123 for good performance on AMD?

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d-netto commented Feb 10, 2023

The number of GC threads can be specified through --gc=N. The previous green check on Test could be misleading, since we are not actually running the GC in parallel (it defaults to single-threaded when --gc is not specified).

It would be nice to run CI with multiple GC threads (worst case, I think we can chose a higher default count of GC threads for testing purposes).

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d-netto commented Feb 10, 2023

Same for PkgEval.

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vchuravy commented Feb 10, 2023

Same for PkgEval.

You can do: runtests(configuration=(julia_flags=["--gc=4"],)) I think. @maleadt is there an option to apply julia_flags to only the current build not the versus build?

@d-netto d-netto force-pushed the dcn/gcthreads branch 3 times, most recently from 5489db2 to 9698e23 Compare February 11, 2023 04:44
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maleadt commented Feb 11, 2023

You can do: runtests(configuration=(julia_flags=["--gc=4"],)) I think. @maleadt is there an option to apply julia_flags to only the current build not the versus build?

The configuration is only applied to the primary build, to influence the versus build you specify vs_configuration.

@d-netto d-netto force-pushed the dcn/gcthreads branch 2 times, most recently from 33137cf to 88ebe01 Compare February 12, 2023 23:11
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vchuravy commented Feb 13, 2023

@nanosoldier runtests(configuration=(julia_flags=["--gc=4"],))

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nanosoldier commented Feb 13, 2023

Your package evaluation job has completed - possible new issues were detected.
A full report can be found here.

@d-netto d-netto force-pushed the dcn/gcthreads branch 4 times, most recently from dc2c305 to 40c9618 Compare February 17, 2023 15:53
@d-netto d-netto marked this pull request as ready for review February 17, 2023 17:32
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@d-netto d-netto force-pushed the dcn/gcthreads branch 4 times, most recently from 3624656 to b5b5791 Compare February 20, 2023 19:50
@d-netto d-netto removed the needs news A NEWS entry is required for this change label Apr 27, 2023
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vchuravy commented Apr 27, 2023

Will merge this tomorrow if there are no further comments.

@d-netto @gbaraldi @vchuravy
Implement parallel marking
6fcf0e7 
Using a work-stealing queue after Chase and Lev, optimized for
weak memory models by Le et al.

Default number of GC threads is half the number of compute threads.

Co-authored-by: Gabriel Baraldi <baraldigabriel@gmail.com>
Co-authored-by: Valentin Churavy <v.churavy@gmail.com>
@vchuravy vchuravy added the merge me PR is reviewed. Merge when all tests are passing label Apr 28, 2023
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KristofferC commented Apr 28, 2023

Just checking if #49545 (comment) should be understood first?

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oscardssmith commented Apr 28, 2023

I don't think so. It appears to be totally unrelated.

@vchuravy vchuravy merged commit abeecee into JuliaLang:master Apr 28, 2023
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@d-netto d-netto removed the merge me PR is reviewed. Merge when all tests are passing label Apr 29, 2023
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Run GC on multiple threads (JuliaLang#48600)
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Run GC on multiple threads (JuliaLang#48600)
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Run GC on multiple threads (JuliaLang#48600)
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Run GC on multiple threads (JuliaLang#48600)
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@vchuravy vchuravy vchuravy approved these changes

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@vtjnash vtjnash Awaiting requested review from vtjnash

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