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/*-
* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
*
* Copyright (c) 2017, Jeffrey Roberson <jeff@freebsd.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice unmodified, this list of conditions, and the following
* disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* $FreeBSD$
*/
#ifndef _SYS_PIDCTRL_H_
#define _SYS_PIDCTRL_H_
/*
* Proportional Integral Derivative controller.
*
* This controller is intended to replace a multitude of threshold based
* daemon regulation systems. These systems produce sharp sawtooths of
* activity which can cause latency spikes and other undesireable bursty
* behavior. The PID controller adapts to changing load conditions and
* adjusts the work done by the daemon to keep a smoother output.
*
* The setpoint can be thought of as a single watermark that the controller
* is always trying to reach. Compared to a high water/low water type
* algorithm the pid controller is dynamically deciding the low water and
* regulating to the high water. The setpoint should be high enough that
* the controller and daemon have time to observe the rise in value and
* respond to it, else the resource may be exhausted. More frequent wakeups
* permit higher setpoints and less underutilized resources.
*
* The controller has been optimised for simplicity of math making it quite
* inexpensive to execute. There is no floating point and so the gains must
* be the inverse of whole integers.
*
* Failing to measure and tune the gain parameters can result in wild
* oscillations in output. It is strongly encouraged that controllers are
* tested and tuned under a wide variety of workloads before gain values are
* picked. Some reasonable defaults are provided below.
*/
struct pidctrl {
/* Saved control variables. */
int pc_error; /* Current error. */
int pc_olderror; /* Saved error for derivative. */
int pc_integral; /* Integral accumulator. */
int pc_derivative; /* Change from last error. */
int pc_input; /* Last input. */
int pc_output; /* Last output. */
int pc_ticks; /* Last sampling time. */
/* configuration options, runtime tunable via sysctl */
int pc_setpoint; /* Desired level */
int pc_interval; /* Update interval in ticks. */
int pc_bound; /* Integral wind-up limit. */
int pc_Kpd; /* Proportional gain divisor. */
int pc_Kid; /* Integral gain divisor. */
int pc_Kdd; /* Derivative gain divisor. */
};
/*
* Reasonable default divisors.
*
* Actual gains are 1/divisor. Gains interact in complex ways with the
* setpoint and interval. Measurement under multiple loads should be
* taken to ensure adequate stability and rise time.
*/
#define PIDCTRL_KPD 3 /* Default proportional divisor. */
#define PIDCTRL_KID 4 /* Default integral divisor. */
#define PIDCTRL_KDD 8 /* Default derivative divisor. */
#define PIDCTRL_BOUND 4 /* Bound factor, setpoint multiple. */
struct sysctl_oid_list;
void pidctrl_init(struct pidctrl *pc, int interval, int setpoint,
int bound, int Kpd, int Kid, int Kdd);
void pidctrl_init_sysctl(struct pidctrl *pc, struct sysctl_oid_list *parent);
/*
* This is the classic PID controller where the interval is clamped to
* [-bound, bound] and the output may be negative. This should be used
* in continuous control loops that can adjust a process variable in
* either direction. This is a descrete time controller and should
* only be called once per-interval or the derivative term will be
* inaccurate.
*/
int pidctrl_classic(struct pidctrl *pc, int input);
/*
* This controler is intended for consumer type daemons that can only
* regulate in a positive direction, that is to say, they can not exert
* positive pressure on the process variable or input. They can only
* reduce it by doing work. As such the integral is bound between [0, bound]
* and the output is similarly a positive value reflecting the units of
* work necessary to be completed in the current interval to eliminate error.
*
* It is a descrete time controller but can be invoked more than once in a
* given time interval for ease of client implementation. This should only
* be done in overload situations or the controller may not produce a stable
* output. Calling it less frequently when there is no work to be done will
* increase the rise time but should otherwise be harmless.
*/
int pidctrl_daemon(struct pidctrl *pc, int input);
#endif /* !_SYS_PIDCTRL_H_ */
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