Plot GC pacer trigger error function

trigger-error.go

// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package main

import (
	"image/color"
	"math"
	"os"

	"github.com/aclements/go-gg/gg"
	"github.com/aclements/go-gg/table"
	"github.com/aclements/go-moremath/vec"
)

func main() {
	var xs, ys []float64
	var fs []color.Color
	for _, x := range vec.Linspace(0, 2, 200) {
		for _, y := range vec.Linspace(0, 1, 200) {
			xs = append(xs, x)
			ys = append(ys, y)
			fs = append(fs, cmap(triggerError(x, y)))
		}
	}

	tab := new(table.Builder).Add("Heap completion ratio (h)", xs).Add("GC CPU (u_a)", ys).Add("fill", fs).Done()
	plot := gg.NewPlot(tab)
	plot.Add(gg.LayerTiles{Fill: "fill"})
	plot.WriteSVG(os.Stdout, 400, 300)
}

func triggerError(completionFraction, utilization float64) float64 {
	// The zero error is not affected by these parameters, but the
	// scale of non-zero errors is.
	const gcpercent = 100
	goalGrowthRatio := float64(gcpercent) / 100
	const gcGoalUtilization = 0.25
	triggerRatio := .95

	// Back out h_a from h, h_T, and h_g.
	actualGrowthRatio := completionFraction*(goalGrowthRatio-triggerRatio) + triggerRatio

	triggerError := goalGrowthRatio - triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-triggerRatio)

	return triggerError
}

func cmap(err float64) color.Color {
	// TODO: Have a diverging color scale.
	maxErr := 0.05
	if err <= 0 {
		v := uint8(255 * math.Max(1-(-err/maxErr), 0))
		return color.NRGBA{255, v, v, 200}
	} else {
		v := uint8(255 * math.Max(1-(err/maxErr), 0))
		return color.NRGBA{v, v, 255, 200}
	}
}