1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2017,  Jeffrey Roberson <jeff@freebsd.org>
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 *    notice unmodified, this list of conditions, and the following
12 *    disclaimer.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 *    notice, this list of conditions and the following disclaimer in the
15 *    documentation and/or other materials provided with the distribution.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
18 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
19 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
20 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
21 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
22 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 */
28
29#ifndef _SYS_PIDCTRL_H_
30#define _SYS_PIDCTRL_H_
31
32/*
33 * Proportional Integral Derivative controller.
34 *
35 * This controller is intended to replace a multitude of threshold based
36 * daemon regulation systems.  These systems produce sharp sawtooths of
37 * activity which can cause latency spikes and other undesireable bursty
38 * behavior.  The PID controller adapts to changing load conditions and
39 * adjusts the work done by the daemon to keep a smoother output.
40 *
41 * The setpoint can be thought of as a single watermark that the controller
42 * is always trying to reach.  Compared to a high water/low water type
43 * algorithm the pid controller is dynamically deciding the low water and
44 * regulating to the high water.  The setpoint should be high enough that
45 * the controller and daemon have time to observe the rise in value and
46 * respond to it, else the resource may be exhausted.  More frequent wakeups
47 * permit higher setpoints and less underutilized resources.
48 *
49 * The controller has been optimised for simplicity of math making it quite
50 * inexpensive to execute.  There is no floating point and so the gains must
51 * be the inverse of whole integers.
52 *
53 * Failing to measure and tune the gain parameters can result in wild
54 * oscillations in output.  It is strongly encouraged that controllers are
55 * tested and tuned under a wide variety of workloads before gain values are
56 * picked.  Some reasonable defaults are provided below.
57 */
58
59struct pidctrl {
60	/* Saved control variables. */
61	int	pc_error;		/* Current error. */
62	int	pc_olderror;		/* Saved error for derivative. */
63	int	pc_integral;		/* Integral accumulator. */
64	int	pc_derivative;		/* Change from last error. */
65	int	pc_input;		/* Last input. */
66	int	pc_output;		/* Last output. */
67	int	pc_ticks;		/* Last sampling time. */
68	/* configuration options, runtime tunable via sysctl */
69	int	pc_setpoint;		/* Desired level */
70	int	pc_interval;		/* Update interval in ticks. */
71	int	pc_bound;		/* Integral wind-up limit. */
72	int	pc_Kpd;			/* Proportional gain divisor. */
73	int	pc_Kid;			/* Integral gain divisor. */
74	int	pc_Kdd;			/* Derivative gain divisor. */
75};
76
77/*
78 * Reasonable default divisors.
79 *
80 * Actual gains are 1/divisor.  Gains interact in complex ways with the
81 * setpoint and interval.  Measurement under multiple loads should be
82 * taken to ensure adequate stability and rise time.
83 */
84#define	PIDCTRL_KPD	3		/* Default proportional divisor. */
85#define	PIDCTRL_KID	4		/* Default integral divisor. */
86#define	PIDCTRL_KDD	8		/* Default derivative divisor. */
87#define	PIDCTRL_BOUND	4		/* Bound factor, setpoint multiple. */
88
89struct sysctl_oid_list;
90
91void	pidctrl_init(struct pidctrl *pc, int interval, int setpoint,
92	    int bound, int Kpd, int Kid, int Kdd);
93void	pidctrl_init_sysctl(struct pidctrl *pc, struct sysctl_oid_list *parent);
94
95/*
96 * This is the classic PID controller where the interval is clamped to
97 * [-bound, bound] and the output may be negative.  This should be used
98 * in continuous control loops that can adjust a process variable in
99 * either direction.  This is a descrete time controller and should
100 * only be called once per-interval or the derivative term will be
101 * inaccurate.
102 */
103int	pidctrl_classic(struct pidctrl *pc, int input);
104
105/*
106 * This controler is intended for consumer type daemons that can only
107 * regulate in a positive direction, that is to say, they can not exert
108 * positive pressure on the process variable or input.  They can only
109 * reduce it by doing work.  As such the integral is bound between [0, bound]
110 * and the output is similarly a positive value reflecting the units of
111 * work necessary to be completed in the current interval to eliminate error.
112 *
113 * It is a descrete time controller but can be invoked more than once in a
114 * given time interval for ease of client implementation.  This should only
115 * be done in overload situations or the controller may not produce a stable
116 * output.  Calling it less frequently when there is no work to be done will
117 * increase the rise time but should otherwise be harmless.
118 */
119int	pidctrl_daemon(struct pidctrl *pc, int input);
120
121#endif	/* !_SYS_PIDCTRL_H_ */
122