Add dead zone ratio to PID controller for noise suppression#1032
Merged
AbdulRahmanAlHamali merged 4 commits intopid-take-2from Feb 27, 2026
Merged
Add dead zone ratio to PID controller for noise suppression#1032AbdulRahmanAlHamali merged 4 commits intopid-take-2from
AbdulRahmanAlHamali merged 4 commits intopid-take-2from
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- Introduced `dead_zone_ratio` parameter in the PID controller to suppress noise from small deviations in error rates. - Updated the `calculate_p_value` method to implement dead zone logic, allowing for more stable control responses. - Enhanced tests to validate the behavior of the dead zone in various scenarios, ensuring it does not impede recovery while effectively filtering noise.
lib/semian/pid_controller.rb
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| delta_error = 0.0 | ||
| else | ||
| # Above dead zone: react to excess only | ||
| delta_error = raw_delta - dead_zone |
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I think the delta_error here should be raw_delta - dead_zone. So that the dead zone does not actually affect calculations: it only silences them when we are within the dead zone, but otherwise everything stays the same
- Updated the handling of the dead zone in the PID controller to allow full signal response above the dead zone, improving control accuracy. - Adjusted comments for clarity regarding the purpose of the dead zone in noise suppression.
AbdulRahmanAlHamali
approved these changes
Feb 27, 2026
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closes https://github.com/Shopify/resiliency/issues/6664
Summary
ideal_error_rate * dead_zone_ratioof the ideal are treated as zero deviation, preventing unnecessary rejection rate increasesHow it works
The dead zone defines a band around the ideal error rate where the controller ignores noise:
0 < delta <= ideal * ratio): delta suppressed to 0delta > ideal * ratio): controller reacts to the excess (delta - dead_zone)Example: with
initial_error_rate: 0.05anddead_zone_ratio: 0.25, error rates up to 6.25% won't trigger rejection.Files changed
lib/semian/pid_controller.rb— dead zone logic incalculate_p_valuelib/semian/adaptive_circuit_breaker.rb— passesdead_zone_ratioto PID controllerlib/semian.rb— wiresdead_zone_ratio: 0.25default in factory methodtest/pid_controller_test.rb— tests for suppression, reaction above threshold, and unimpeded recovery