runtime:change gcBackgroundUtilization optional

[lxq015]

Port of https://gitcode.com/openeuler/golang/pull/70 to the ZTE RISC-V toolchain.

The concurrent GC previously reserved a fixed 25% of CPU for background marking. This change introduces the GOGCRATIO environment variable (1–99, default 25) to let users trade mutator CPU for marking throughput per workload.

Implementation: gcController.gcRatio stores the configured utilization; readGOGCRATIO parses and clamps to [0.01, 0.99]; pacer (startCycle/endCycle) and GC CPU limiter (mgclimit) use the dynamic value. Tests updated; new benchmark BenchmarkGCBackground added.

Benchmarks

• goos: linux
• goarch: riscv64
• cpu: Spacemit(R) X60
• GOGC regimes: 100 (default), 20 (moderate), 5 (dense)
• GOMAXPROCS: 1, 4, 8
• Baseline: GOGCRATIO=25

Benchstat comparisons below:

GOGCRATIO=25 vs 50

                                     │ GOGCRATIO=25 │              GOGCRATIO=50               │
                                     │   sec/op    │   sec/op     vs base                     │
GCBackgroundDefault/gcbound            2.938µ ± 2%   3.076µ ± 4%  +4.68% (p=0.002 n=6)
GCBackgroundDefault/gcbound-large      2.949µ ± 4%   3.140µ ± 3%  +6.48% (p=0.009 n=6)
GCBackgroundDefault/gcbound-dense      3.232µ ± 3%   3.599µ ± 5% +11.37% (p=0.002 n=6)
GCBackgroundModerate/gcbound           7.756µ ± 8%   6.483µ ±40%        ~ (p=0.065 n=6)
GCBackgroundModerate/gcbound-large     7.231µ ±22%   7.601µ ±17%        ~ (p=0.093 n=6)
GCBackgroundModerate/gcbound-dense     7.869µ ±18%   7.567µ ±11%        ~ (p=0.310 n=6)
GCBackgroundDense/gcbound              9.329µ ± 5%   9.341µ ±20%        ~ (p=0.937 n=6)
GCBackgroundDense/gcbound-large        9.582µ ± 7%  12.885µ ±32%        ~ (p=0.065 n=6)
GCBackgroundDense/gcbound-dense        9.517µ ±11%   9.763µ ±23%        ~ (p=0.310 n=6)
GCBackgroundDefault/gcbound-4          1.885µ ± 2%   1.891µ ± 1%        ~ (p=0.329 n=6)
GCBackgroundDefault/gcbound-large-4    1.929µ ± 2%   1.921µ ± 1%        ~ (p=0.699 n=6)
GCBackgroundDefault/gcbound-dense-4    1.951µ ± 2%   1.982µ ± 2%        ~ (p=0.394 n=6)
GCBackgroundModerate/gcbound-4         3.886µ ± 5%   3.645µ ±12%        ~ (p=0.132 n=6)
GCBackgroundModerate/gcbound-large-4   3.703µ ± 2%   3.385µ ± 1%  -8.59% (p=0.002 n=6)
GCBackgroundModerate/gcbound-dense-4   4.361µ ± 7%   3.495µ ± 2% -19.86% (p=0.002 n=6)
GCBackgroundDense/gcbound-4           10.655µ ±13%   6.684µ ± 6% -37.27% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-4      8.588µ ±14%   5.315µ ± 1% -38.11% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense-4      9.318µ ±12%   5.544µ ± 3% -40.50% (p=0.002 n=6)
GCBackgroundDefault/gcbound-8          1.852µ ± 1%   1.862µ ± 2%        ~ (p=0.582 n=6)
GCBackgroundDefault/gcbound-large-8    1.894µ ± 1%   1.889µ ± 3%        ~ (p=0.937 n=6)
GCBackgroundDefault/gcbound-dense-8    1.925µ ± 2%   1.901µ ± 2%  -1.27% (p=0.002 n=6)
GCBackgroundModerate/gcbound-8         3.490µ ± 1%   3.257µ ± 8%  -6.68% (p=0.028 n=6)
GCBackgroundModerate/gcbound-large-8   3.161µ ± 7%   3.045µ ± 2%  -3.69% (p=0.017 n=6)
GCBackgroundModerate/gcbound-dense-8   3.652µ ± 3%   3.223µ ± 1% -11.75% (p=0.002 n=6)
GCBackgroundDense/gcbound-8            8.290µ ±15%   5.929µ ± 2% -28.48% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-8      5.859µ ± 7%   5.039µ ± 2% -13.99% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense-8      7.406µ ± 6%   5.769µ ± 8% -22.10% (p=0.002 n=6)
geomean                                4.482µ        4.096µ        -8.61%

(All B/op and allocs/op unchanged at 832.0 B and 3.000 allocs/op across all benchmarks.)

GOGCRATIO=25 vs 75

                                     │ GOGCRATIO=25 │              GOGCRATIO=75               │
                                     │   sec/op    │   sec/op     vs base                     │
GCBackgroundDefault/gcbound            2.938µ ± 2%   3.242µ ± 4% +10.33% (p=0.002 n=6)
GCBackgroundDefault/gcbound-large      2.949µ ± 4%   3.226µ ± 9%  +9.41% (p=0.002 n=6)
GCBackgroundDefault/gcbound-dense      3.232µ ± 3%   3.633µ ± 1% +12.42% (p=0.002 n=6)
GCBackgroundModerate/gcbound           7.756µ ± 8%   7.407µ ±30%        ~ (p=0.310 n=6)
GCBackgroundModerate/gcbound-large     7.231µ ±22%   8.415µ ± 7% +16.37% (p=0.002 n=6)
GCBackgroundModerate/gcbound-dense     7.869µ ±18%   8.585µ ± 6%        ~ (p=0.093 n=6)
GCBackgroundDense/gcbound              9.329µ ± 5%  10.491µ ± 6% +12.46% (p=0.002 n=6)
GCBackgroundDense/gcbound-large        9.582µ ± 7%  12.307µ ±13% +28.44% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense        9.517µ ±11%  10.789µ ± 3% +13.37% (p=0.004 n=6)
GCBackgroundDefault/gcbound-4          1.885µ ± 2%   1.881µ ± 1%        ~ (p=0.485 n=6)
GCBackgroundDefault/gcbound-large-4    1.929µ ± 2%   1.919µ ± 2%        ~ (p=0.485 n=6)
GCBackgroundDefault/gcbound-dense-4    1.951µ ± 2%   1.938µ ± 3%        ~ (p=0.509 n=6)
GCBackgroundModerate/gcbound-4         3.886µ ± 5%   3.217µ ±21% -17.22% (p=0.015 n=6)
GCBackgroundModerate/gcbound-large-4   3.703µ ± 2%   3.206µ ± 1% -13.42% (p=0.002 n=6)
GCBackgroundModerate/gcbound-dense-4   4.361µ ± 7%   3.261µ ± 1% -25.22% (p=0.002 n=6)
GCBackgroundDense/gcbound-4           10.655µ ±13%   5.290µ ± 5% -50.35% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-4      8.588µ ±14%   4.557µ ± 3% -46.94% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense-4      9.318µ ±12%   4.626µ ± 4% -50.36% (p=0.002 n=6)
GCBackgroundDefault/gcbound-8          1.852µ ± 1%   1.868µ ± 2%        ~ (p=0.394 n=6)
GCBackgroundDefault/gcbound-large-8    1.894µ ± 1%   1.880µ ± 1%  -0.74% (p=0.028 n=6)
GCBackgroundDefault/gcbound-dense-8    1.925µ ± 2%   1.879µ ± 1%  -2.39% (p=0.002 n=6)
GCBackgroundModerate/gcbound-8         3.490µ ± 1%   3.057µ ± 4% -12.39% (p=0.002 n=6)
GCBackgroundModerate/gcbound-large-8   3.161µ ± 7%   2.991µ ± 2%  -5.39% (p=0.004 n=6)
GCBackgroundModerate/gcbound-dense-8   3.652µ ± 3%   3.087µ ± 3% -15.46% (p=0.002 n=6)
GCBackgroundDense/gcbound-8            8.290µ ±15%   5.437µ ± 7% -34.42% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-8      5.859µ ± 7%   4.737µ ± 3% -19.13% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense-8      7.406µ ± 6%   5.320µ ± 6% -28.17% (p=0.002 n=6)
geomean                                4.482µ        4.007µ       -10.60%

(B/op and allocs/op unchanged.)

GOGCRATIO=25 vs 100

                                     │ GOGCRATIO=25 │              GOGCRATIO=100              │
                                     │   sec/op    │   sec/op     vs base                     │
GCBackgroundDefault/gcbound            2.938µ ± 2%   3.077µ ± 3%  +4.70% (p=0.002 n=6)
GCBackgroundDefault/gcbound-large      2.949µ ± 4%   3.209µ ±10%         ~ (p=0.065 n=6)
GCBackgroundDefault/gcbound-dense      3.232µ ± 3%   3.616µ ± 5% +11.90% (p=0.002 n=6)
GCBackgroundModerate/gcbound           7.756µ ± 8%   6.214µ ± 5% -19.88% (p=0.002 n=6)
GCBackgroundModerate/gcbound-large     7.231µ ±22%  10.961µ ±14% +51.58% (p=0.002 n=6)
GCBackgroundModerate/gcbound-dense     7.869µ ±18%   8.985µ ±18% +14.18% (p=0.026 n=6)
GCBackgroundDense/gcbound              9.329µ ± 5%  10.870µ ± 9% +16.52% (p=0.002 n=6)
GCBackgroundDense/gcbound-large        9.582µ ± 7%  22.967µ ±10% +139.69% (p=0.002 n=6)
GCBackgroundDense/gcbound-dense        9.517µ ±11%  16.279µ ±23% +71.06% (p=0.002 n=6)
GCBackgroundDefault/gcbound-4          1.885µ ± 2%   2.031µ ± 2%  +7.77% (p=0.002 n=6)
GCBackgroundDefault/gcbound-large-4    1.929µ ± 2%   2.077µ ± 3%  +7.70% (p=0.002 n=6)
GCBackgroundDefault/gcbound-dense-4    1.951µ ± 2%   2.197µ ± 3% +12.58% (p=0.002 n=6)
GCBackgroundModerate/gcbound-4         3.886µ ± 5%   3.911µ ± 5%         ~ (p=0.589 n=6)
GCBackgroundModerate/gcbound-large-4   3.703µ ± 2%   4.149µ ± 4% +12.06% (p=0.002 n=6)
GCBackgroundModerate/gcbound-dense-4   4.361µ ± 7%   4.768µ ± 5%  +9.32% (p=0.004 n=6)
GCBackgroundDense/gcbound-4           10.655µ ±13%   8.416µ ±11% -21.01% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-4      8.588µ ±14%   8.341µ ±25%         ~ (p=0.589 n=6)
GCBackgroundDense/gcbound-dense-4      9.318µ ±12%  11.229µ ± 9% +20.52% (p=0.009 n=6)
GCBackgroundDefault/gcbound-8          1.852µ ± 1%   1.901µ ± 2%  +2.67% (p=0.013 n=6)
GCBackgroundDefault/gcbound-large-8    1.894µ ± 1%   1.974µ ± 6%  +4.25% (p=0.002 n=6)
GCBackgroundDefault/gcbound-dense-8    1.925µ ± 2%   1.993µ ± 4%  +3.53% (p=0.002 n=6)
GCBackgroundModerate/gcbound-8         3.490µ ± 1%   3.210µ ± 1%  -8.02% (p=0.002 n=6)
GCBackgroundModerate/gcbound-large-8   3.161µ ± 7%   3.354µ ± 3%  +6.11% (p=0.015 n=6)
GCBackgroundModerate/gcbound-dense-8   3.652µ ± 3%   3.716µ ± 3%         ~ (p=0.485 n=6)
GCBackgroundDense/gcbound-8            8.290µ ±15%   5.895µ ± 4% -28.89% (p=0.002 n=6)
GCBackgroundDense/gcbound-large-8      5.859µ ± 7%   5.376µ ±12%         ~ (p=0.065 n=6)
GCBackgroundDense/gcbound-dense-8      7.406µ ± 6%   7.302µ ± 7%         ~ (p=0.699 n=6)
geomean                                4.482µ        4.861µ        +8.44%

(B/op and allocs/op unchanged.)

Conclusion

The knob works as intended:

Default GOGC=100 (common case): raising to 50/75 does not hurt on multi-core; only 100 causes slowdown due to GC CPU limiter engagement, confirming no regression.

GC-bound workloads (GOGC=5, dense): 75 is near-optimal, cutting time by 19–50% on 4–8 cores; 100 gives back some gain (limiter kicks in), 25 is under-provisioned.

Single core: higher GOGCRATIO always hurts (up to +140% at 100) because no parallelism exists for extra mark workers.

Geomean across all 27 configs: 25→50 -8.61%, 25→75 -10.60%, 25→100 +8.44%. The GC CPU limiter provides a safety ceiling against oversubscription.

[BoyaoWang430]

You can read the Garbage collection part of this document first. https://github.com/golang/proposal/blob/master/design/48409-soft-memory-limit.md

This implementation allows GOGCRATIO values from 1% to 99%, while the GC CPU limiter is designed around a 50% threshold. Above 50%, background mark workers alone can keep GC CPU usage over the limit. The limiter only disables GC assists; it does not reduce dedicated mark workers.

[lxq015]

You can read the Garbage collection part of this document first. https://github.com/golang/proposal/blob/master/design/48409-soft-memory-limit.md

This implementation allows GOGCRATIO values from 1% to 99%, while the GC CPU limiter is designed around a 50% threshold. Above 50%, background mark workers alone can keep GC CPU usage over the limit. The limiter only disables GC assists; it does not reduce dedicated mark workers.

@BoyaoWang430 Thanks for the information and the reference. This PR is backported from the openEuler(https://gitcode.com/openeuler/golang/pull/70). Would it be acceptable to tighten the upper limit of GOGCRATIO from 99 to 50, to keep it consistent with the GC CPU limiter's 50% threshold?

[BoyaoWang430]

You can read the Garbage collection part of this document first. https://github.com/golang/proposal/blob/master/design/48409-soft-memory-limit.md
This implementation allows GOGCRATIO values from 1% to 99%, while the GC CPU limiter is designed around a 50% threshold. Above 50%, background mark workers alone can keep GC CPU usage over the limit. The limiter only disables GC assists; it does not reduce dedicated mark workers.

@BoyaoWang430 Thanks for the information and the reference. This PR is backported from the openEuler(https://gitcode.com/openeuler/golang/pull/70). Would it be acceptable to tighten the upper limit of GOGCRATIO from 99 to 50, to keep it consistent with the GC CPU limiter's 50% threshold?

With the existing rounding rule and its 30% relative-error tolerance, 38% is the highest integer setting that guarantees dedicated workers use strictly less than half of the P